Getting confirmation of a previous overpayment

Is it possible to see how much you have in your 'account' on gov.uk after an overpayment last year. For long and boring reasons I never tried to claim it back, and now want to pay this years bill less whatever is in there. I've tried to understand it from my self assessment account, but come away more confused than ever. Is the only way to get a nice simple number really to ring them up?