Andrew Mountbatten-Windsor barely had time to enjoy his morning tea and Abernethy biscuits before Thames Valley Police turned up on his doorstep that day back in February.

Despite receiving a tip-off that detectives were heading his way, nothing could have prepared him for the shock of seeing six unmarked cars pulling up outside Wood Farm at around 8am on his 66th birthday.

While the erstwhile Duke of York was arrested and taken away for questioning to a Norfolk police station, plain-clothed officers, armed with a search warrant, rummaged through the rooms of his temporary home on the Sandringham Estate.

At the same time, 140 miles away, their colleagues searched Mountbatten-Windsor’s old residence, Royal Lodge in Windsor, removing items including paperwork and electronic equipment left behind when he was hastily sent into exile the previous month.

For the son of a monarch, even an abased one, this was a humiliating turn of events indeed and one which, it is now clear, was a step too far for a man who at birth was second in line to the British throne.

Infuriated by what he saw as an outrageous invasion of his privacy, Mountbatten-Windsor boldly challenged the legality of the warrants used to trawl through his homes, becoming the first member of the Royal Family to challenge British criminal proceedings since Charles I at his trial for high treason in 1649.

Unlike his ill-fated Stuart ancestor, however, Mountbatten-Windsor’s complaint has been upheld.

A High Court judge has declared that the warrants issued at the Old Bailey to Thames Valley Police while they investigated allegations of misconduct in public office were indeed illegal because the wrong legislation was applied to them.

They were quashed in July in a behind-closed-doors hearing which was finally made public this week.

But if this surprise court victory comes down to little more than a legal technicality then Mountbatten-Windsor’s willingness to take on the police suggests a newfound fighting spirit in a man said to have been left broken by his devastating fall from grace.

‘Something of the old Andrew has returned. The spirit of the grand old Duke of York has been rekindled,’ declared a royal source speaking exclusively to the Daily Mail this week.

‘He believes he is an honourable man who has been unfairly treated. He is determined not to be just a punchbag or a scapegoat. He is sick and tired of his lot and has started to fight back.’

Is it possible then, that this week’s legal drama is just the tip of the iceberg and that, a year to the month since he was stripped of his titles and reduced to the rank of commoner, Mountbatten-Windsor is on the brink of an extraordinary comeback?

For there are other signs that the tide is turning for the beleaguered former HRH, not least the shifting mood among north Norfolk’s upper-crust set – the so-called Turnip Toffs – who appear to have growing sympathy for a man who, until recently, was regarded as a social pariah.

As The Mail on Sunday revealed last week, having been cast into the wilderness since his reduction in rank, Andrew’s phone has been ringing more frequently of late, with a steady stream of invitations from well-heeled neighbours to cosy kitchen suppers and casual barbecues.

While some of them are said to enjoy ‘trying to shock’ their guests by including the scandal-plagued former prince among their number, according to one society figure: ‘Things have gone too far. The feeling is that Andrew needs to be welcomed back, at least to some extent.’

Spurred on by the ever-louder whispers that he may well soon be off the legal hook, recent hosts are said to include the Earl and Countess of Leicester – better known to Andrew as Tom and Polly – who have apparently had him round for an informal dinner at Holkham Hall, their grand Norfolk estate.

According to the Daily Mail’s source these intimate soirees, which are aimed at bringing reclusive Andrew ‘out of himself’, are kept deliberately casual – for the time being at least.

‘Anything formal would be fraught with difficulties when it comes to things like table plans,’ says the source, ‘because Andrew, who would once have been guest of honour, is now outranked by many in the Turnip Toff set.’

But such is the growing mood of confidence around Mountbatten-Windsor, who has always denied any wrongdoing and has not yet been charged with any criminal offence, that some of his friends have now urged him to go further and mount a legal challenge against the King’s decision to remove his titles and royal status.

‘He remains the son, brother and uncle of future monarchs,’ says the royal source. ‘Most people don’t realise that technically he is still the Duke of York – he still insists on being called "Sir" and "Duke" at home. Only an Act of Parliament can extinguish his title.’

Encouraging, too, is the steady flow of ‘fan mail’ now being delivered to Marsh Farm which has been Mountbatten-Windsor’s home on the Sandringham estate since early April.

Most of the letters are opened by his part-time secretary who sends a formal acknowledgment to well-wishers by way of reply.

This month he handed back the keys to Royal Lodge, along with £1.5million to pay the Crown Estate for repairs to the main house and a dilapidated outbuilding.

In receipt of a generous stipend from the King’s private funds – on top of the modest Royal Navy pension he receives – Mountbatten-Windsor clearly isn’t short of cash although his means of support, as always, are pretty much invisible.

In recent months, the King’s attitude to his troublesome young brother also appears to have softened. While some courtiers had advised that his movements in exile should be restricted, King Charles has given his sibling more freedom, allowing him to come and go as he pleases – albeit with a security detail to keep track of his movements.

The father of two is also allowed to ride, well away from the eyes of the public, on designated areas of the monarch’s privately owned Sandringham estate.

This summer has seen the brothers speaking by phone. Andrew has also received visits from his other siblings, Edward and Anne, both of whom are said to be concerned about his harsh treatment and the lack of emotional support offered to their brother as his life fell apart.

While steadfastly loyal to the monarchy, Andrew is also said to feel ‘badly let down’ by the King who, along with the late Queen, pushed him to settle with the late Virginia Giuffre following her claims to have had sex with him as a 17-year-old after being trafficked to the UK by Epstein’s girlfriend Ghislaine Maxwell.

Concerned about the impact Giuffre’s claims would have on the impending Platinum Jubilee, it is said that the Queen and the then Prince of Wales hoped a financial settlement would draw a line under the scandal.

‘Andrew didn’t want to do it and was irate about not holding out,’ says the royal source. ‘The plan backfired. His daughters weren’t the only ones who wanted to know why he was settling with a woman he claimed never to have met.’

His supporters also contrast his harsh treatment with that meted out to Prince Harry, who has spoken out against his relatives and the monarchy – not least in his best-selling autobiography Spare – and has suffered scarcely any consequences.

Meanwhile Andrew, it is pointed out, has remained loyal – and tight-lipped.

According to the source: ‘He always has felt that his nephew has been treated leniently compared to himself, especially as Andrew has remained loyal to the throne and would never do anything that would cause trouble. That’s very unlike Harry and Meg’s stone-throwing and tittle-tattling across the Pond.

‘The wider family feels he deserves credit for not rocking the royal boat.’

‘He would love to go further as he seeks to rehabilitate himself,’ says the Daily Mail’s royal source.

‘Although he knows that his hopes of riding again in Trooping the Colour are gone he wants to be accepted back into the family rather than be hidden away as a pariah.’

The real test will come at Christmas when the Royal Family gather at Sandringham.

‘The true extent as to how far he is still being frozen out by his own family should become clear at Christmas where the optics of an isolated Andrew whilst the rest of the family party nearby is causing courtiers some jitters,’ adds the source.

Another hurdle lying just around the corner is next month’s Commonwealth summit meeting in Antigua where the question of whether to remove Mountbatten-Windsor – who is still eighth in line to the throne, after Prince Harry’s daughter Princess Lilibet – from the royal succession is due to be discussed.

The matter can only be decided by all 15 Commonwealth states who would have to pass legislation in their own parliaments to effect any change.

Much, of course, will depend on what happens next in the police investigation.

This week Thames Valley Police blamed the courts for the embarrassing search warrant muddle and, having applied to keep the material seized during those searches, insist ‘it is not a judgement on our ongoing investigation’.

Some legal experts believe, however, that the unlawful searches of Andrew’s residences now cast doubt on the entire police investigation against him.

Part of Andrew’s gripe, the Daily Mail has been told, was that the search of Royal Lodge, which took much longer than Wood Farm, was undertaken without him or a representative present.

He also complained that detectives took private and confidential items – belonging to both him and his ex-wife Sarah Ferguson – which apparently had no bearing on their investigation.

‘The police appear to have made no distinction between the two,’ said the source.

Marcus Johnstone of PCD Solicitors, which specialises in criminal defence, told the Daily Mail this week that it was ‘concerning that Thames Valley Police did not make sure that all their ducks were in a row before pulling the trigger’.

He added: ‘As things stand, what the force has now opened itself up to is the possibility of further legal action and the collapse of the case against Mr Mountbatten-Windsor entirely.’

Even without this legal setback, misconduct in public office is notoriously difficult to prosecute largely because there is no definitive list of positions that qualify for that description.

Police will have to interview numerous officials and seek disclosure from the Government of any documents Andrew signed regarding confidentiality in his role as British trade envoy which he held for ten years until 2011.

They would need to show that any misconduct was linked to the role itself and that Andrew ‘deliberately’ did something he knew was wrong or acted with ‘reckless indifference’.

Early signs that the police were struggling with the case emerged in May this year when Thames Valley Police broadened their ‘misconduct in public office’ investigation to include potential offences including sexual misconduct and corruption.

Their public appeal for ‘victim-survivors’ to come forward was interpreted by some as evidence of growing desperation.

Assistant Chief Constable Oliver Wright said the force had assembled a dedicated team of its ‘most experienced specialist investigators’, which include detectives specialising in serious sexual attacks and financial experts.

Thames Valley Police, which has been liaising with the US Department of Justice following the release of millions of documents from the Epstein Files, has declined to say how much has been spent on their investigation but given the senior level of officers and considerable resources allocated so far, the costs will be mounting.

Given the significance of this high-profile case, any decision on whether to prosecute is likely to be taken by the Director of Public Prosecutions, Stephen Parkinson. As things stand, there is no sign of that happening any time soon.

A year on from one of the most devastating downfalls in the history of the British monarchy, an increasingly defiant Mountbatten-Windsor appears in no mood to go quietly.