A few years back I got notice of a statutory fine of 100 quid for failing to send in a tax return. I called them and said I would have returned it if they'd sent it to me in the first place. They said they did - they sent it to RAF Innsworth. I said I'd never even visited the place, let alone lived there, so they waived the fine but said I had to fill in a form and calculate the tax myself as it was now too late to have them do it for me.
I filled in the form, adding my Gran's age to my birthday and trying to follow their instructions. At the end, I calculated they owed me about 900 quid so I sent that in thinking they'll check it and find out we were in fact even (There's nothing complicated in my return - PAYE plus an AVC). I got a cheque from them!
Next year, as I'm filling out the new form, I get a phone call from the Inland Revenue auditing office asking me why I thought they owed me the money the previous year. I said because when I filled their stupid form in, that's what it came out as!. They said OK. I calculated the tax myself again and found out they owed me about 600 quid and got another cheque!
This year I calculated it myself and we were about even, but subsequently got notification that I owed them many thousands! I couldn't find the calculation booklet but just applied the basic allowances from the IR site to my pay and thought I was still correct (I am now an ex-pat overseas but paying full UK tax as if I lived there). Phoned them and they said you didn't fill in the box for the AVC. Luckily I had copied the return and could prove that I had. They then said I couldn't have the normal intiial tax free part as I hadn't ticked the appropriate box. Again, I could prove that I had.
This has happened to a lot of people I know this year and I suspect it's down to the IR employing temporary staff to complete the tax return or just a very incompetent workforce. But the moral of the story, is don't assume, check!