Well obviously, but with everything else going on, it just doesn't happen.
Now you know that's rubbish; you've got to do it at some stage; and you know you could've done it earlier if you'd had made the effort; you're not telling me you couldn't have spent a day-or-two doing it when you've been rained/frozen off?
Look, get your paper self assessment form and read it. In there it will tell you how your tax is caluclated; I've just been looking for mine; the notes; but can't find it at the moment.
But go through it carefully with a cup of coffee; it's easy.
Once you work it out, if your total tax bill for 07/08 is for eg. £1000 and you've paid £800 on account, then you owe £200, but will have to pay on account for 08/09 another £1000 in two installments. Does that make sense?
Get your tax statements together too; go through them; however it is possible they've made a mistake, but I doubt it; but it'll be a bugger trying to get through to the self-assessment help-line at this time of year.