Forget the family fall-outs and problematic relatives, what we saw unfold in the Caribbean over the past week proved why, for the time being at least, monarchy matters.

While most of the headlines focused on the thorny issue of reparations and an unfortunate lack of umbrellas, behind the scenes, the King's mission to win hearts, minds – and, more importantly, trade deals – was quietly considered by diplomats to be a royal tour de force.

One senior palace aide went so far as to remark last night that Charles was 'able to reach the parts that, dare I say, politicians probably couldn't quite have reached'.

In fact, you might argue, it was the flat-footed, ill-timed and even worse-thought-out manoeuvrings of our elected political leaders that actually risked derailing the meticulously planned enterprise.

The King, who turns 78 next week and, we should not forget, is still receiving cancer treatment, undertook 34 public engagements, often in arduous heat and humidity.

His stamina has been little short of incredible, shaking hundreds of hands as he ploughed through walkabouts, attending scores of ceremonies and high-level bilateral meetings – and even spontaneously joining in the dancing, playing a set of cowbells, at a Bahamian 'junkanoo' parade that left his protection officers agog.

Senior royal aides say this is what he 'lives for'. Asked whether the sovereign had any limitations, one source said: 'I think probably the best response to that is a picture of a man about to turn 78 jangling the cowbells as he jives down the street in nearly 40-degree heat.

'When the next engagement finished, I got in the car next to him and said, 'Crikey, I'm hot'! He said, 'Oh never mind'. So no, he copes. I mean, frankly, he copes better with it than I do, and I'm younger than him.

'This is literally what he was born to do, he's very good at it, and he enjoys the reaction he gets from people in the course of doing it. 

'The King is not just King for the easy parts of the job, but the tough parts too. In fact, that's probably the part of the job that he relishes most, whether that's leading on the environment, whether it's renewing and refreshing and reinvigorating relationships with nations where there have been some complex political issues at play, that is what he lives and breathes for.'

One aide tells me the King worked out when he was a young man that the 'women in his family were very long-lived and that he would be Prince of Wales for a very long time'. It's why he decided to try to carve out a proactive and meaningful role for himself, an approach he has sought to replicate as King. In other words the job title may be new but the way he has tackled the role hasn't changed.

And the long years of his apprenticeship have not dimmed his enthusiasm either.

Indeed, sources say his energy remains infectious (on the return flight home he was still working through his red boxes and starting to write some of the 8,000 Christmas cards he and his wife send out each year) and he relishes the challenges of the job.

In fact those closest to him say he actively prefers those trickier moments, citing this year's state visit to the US and his post-Brexit European tours as examples.

'We're just in the fifth year of the reign and I think what we are seeing is a very clear pattern emerging of the Government asking the King to head into quite politically charged situations, quite challenging situations,' a senior aide reveals.

'I saw a number of reports before the visit that this would be the greatest test of his reign, and I smiled a little bit because I seem to remember everyone said that about the last visit.

'But I think it's right, because that is exactly the pattern that we're now seeing.'

The source continued: 'I would say, in the round, this has been a complex visit. It's been challenging. It's been exhausting. 

But the takeaway is that it's been very successful, and the King has demonstrated his statecraft both by word and deeds. 

It's a great secret weapon to send the King. He is able to reach the parts that, dare I say, politicians probably couldn't quite have reached. We return home with heads held high, including that which bears the Crown.'

Inevitably on these sorts of trips there is a great deal of focus on the start and the finish: in this case the infectious joy of the Bahamas and the elemental and political challenges of Antigua and Barbuda.

But for diplomats it was in Guyana, in the middle, where the King was most effective, having already formed a warm relationship with the president of the country that boasts the world's fastest growing economy.

Those close to the monarch say the secret to his success, especially on slightly 'spicy' trips such as these, is stamina, stoicism and humour because, of course, 'not everything goes right'. 'It's how you deal with them in the moment that matters,' they said.

Of course the most notable 'moment' of the King's tour was 'umbrella-gate', which saw him caught on camera angrily gesticulating at staff after they failed to bring him a brolly sufficiently quickly in a downpour, leaving him to undertake several hours of ceremonial duties in a soaking wet, see-through summer suit.

'You know sometimes things don't work entirely according to plan, but it was very quickly resolved and, as we saw, he was laughing,' the source said.

Sources say the Queen is instrumental to her husband's happiness.

'It's the good humour that gets them both through it,' an aide said.

'I know at the end of the day, and sometimes at the end of each engagement, they have a good laugh together about where things have gone [wrong].

'It may have gone a bit wonky, but fundamentally underpinning it is that strength of their relationship and their determination to keep cracking on despite age and circumstance.