HMRC are a law unto themselves and although I agree with you in principle Bluebell it's possible to come unstuck even when you have acted honestly and in good faith.
I was self-employed for over 20 years, employed an accountant, paid my tax in full and on time every year.
I was surprised a few months ago to receive a demand from them for 'interest'.
I rang up and spoke to a very young man who explained that yes, I had certainly paid in full etc but HMRC had retrospectively revised the amount I should have paid, and therefore although I had paid the correct amount at the time, I now owed them interest on what they had later decided I should have paid. This was later reiterated in a fairly heated correspondence.
It turned out that my accountant had underestimated the amount I should have paid 'on account'. That means the tax that self employed people have to pay on money they haven't yet earned.
As it was only a small sum (£17.60) I just paid and didn't take it any further, but i felt it was quite wrong in principle that they can just alter assessments retrospectively like that. If it had been £176 or £1760 I would have been none the wiser and I would certainly have challenged it higher up the feeding chain.
It left me with the deep suspicion that HMRC are just trying it on with as many people as possible, over small amounts that are not going to be challenged, rather like criminals who manage to deduct 10p from millions of bank accounts - it all adds up to rather a nice sum.