Today's column is the 500th that Steve Webb has written for This is Money. Watch out for a series of articles and events to celebrate this landmark all this week.

I have emailed, written and called the Department for Work and Pensions several times over the last few years about my 87-year-old mum's pension.

She has Alzheimer's so I am doing it for her. I have asked the DWP to check whether she is owed any money as she received a very small pension for many years.

I have heard nothing and each time I phone they say I have to wait as reviews are still being made.

Is this correct or have all the reviews by now been made and my mum or at least my correspondence has been overlooked?

Over the course of writing this column for more than ten years, I have tried to cover a wide range of pension topics.

But it is probably writing about the state pension system that has had the biggest impact.

And so it is fitting that, as I sit down to write column 500, I should provide an update for you and other readers on where we have got to on getting corrections for the many state pension errors we have unearthed.

As a reminder, there were three types of cases where we found DWP had made errors affecting tens of thousands of people in each case.

In all these cases, something changed in the life of a retiree which should have prompted a state pension reassessment, but that reassessment never happened. The three groups were as follows.

- Married women with a modest National Insurance record (under 60 per cent) in their own right, who were entitled to an uplift to the 60 per cent rate when their husband retired, but where this never happened. This only applied when the husband reached pension age from March 17, 2008 onwards.

- Widows whose state pension should have been reassessed when their husband died but where this never happened. Some widowers were also included in this group.

- Over-80s who had a modest NI record (under 60 per cent) and who should have switched automatically from this low pension to the 60 per cent 'Category D' rate for those aged 80 or over, but where this never happened.

Initially, DWP claimed that these were just isolated errors and fixed cases one by one, but eventually we persuaded the department that there were systematic problems which needed to be fixed.

In response, it launched a four-year correction programme, starting in January 2021, which checked nearly three-quarters of a million pension assessments.

That process ended in early 2025.

In total, DWP found errors in around 130,000 cases and had to pay out over £800million in arrears.

The information below shows the number of errors in each of the three categories, together with the average and total amount of arrears paid in each case. The figures are as at March 2025, and are the most recent available.

Married women: 47,004 underpayments; £5,553 average arrears; £253million total amount repaid.

Widowed people: 50,261 underpayments; £11,725 average arrears; £483million total amount repaid.

33,683 underpayments; £2,203 average arrears; £69million total amount repaid.

As this shows, the biggest losses were generally suffered by widows, some of whom missed out on many years of increased pension.

Although the average loss was a little over £11,000, we saw some extreme arrears payments, in some cases exceeding £100,000.

This correction exercise is now complete as far as DWP is concerned, so it is unlikely that anyone waiting to hear the outcome of a review will get an uplift.

In addition, DWP's general policy was that even if it did review a case but found no error it didn't write to notify people.

If you still think that your mother might have fallen into one of the categories described above, do let me have details and I'm happy to see if she might have 'fallen through the net' when it came to checks.

There was however an extraordinary further development in this whole saga.

Not unreasonably, DWP eventually asked itself why all of these errors had gone undetected – in some cases for decades – and concluded that its checks on state pension calculations weren't good enough.

So, a few years ago, it decided to do more thorough regular checks and found another category of error, potentially affecting tens of thousands more women – not all of whom have yet retired.

The group missing out was mothers (and some fathers) who should have received 'Home Responsibilities Protection' for time at home raising a child aged under 16 for a full year from April 1978 onwards.

HRP had the effect of boosting the NI record of those who weren't paying NI because of caring responsibilities.

I have written about this in more detail in an earlier column, but the key point is that a process is now underway to track down these women. Because the issue is that the HRP is missing from their NI records, DWP cannot simply identify the problem cases and fix them.

HMRC (which keeps NI records) has been writing to women it thinks might have been affected.

The letters which have been sent to date have triggered around 12,000 further arrears payments worth £104million in total, as at March 2025.

However, you do not need to wait for an HMRC letter. Instead, you can check if you might be entitled to HRP here and, if eligible, you can apply here.

I would like to thank everyone who has written in with the first 500 questions which I have answered – do please keep them coming!

And a special thank you to those who first alerted us to the state pension problems, which enabled us to put pressure on DWP to get them fixed, to the benefit of over 130,000 retirees.