I once had a tax assessment on my pension that concluded I owed 9p. Verbally, but they refused to put it in writing, I was told to ignore it. I sent a cheque for 10p with a note to keep the change. I got a giro for 1p back with a note that it was no longer allowed to run a tax account in credit.
A retired tax inspector told me some years later that the 9p outstanding could be used, if it was a bit quiet, or there was a new member of staff, or if there was a pain in the arse on the staff, to send that member of staff out to go through my books with a fine tooth comb, even though my "books" were PAYE taxed pensions!