I had a similar situation some years ago, and this might be your problem. After window cleaning as a second job for a number of years, I was made redundant and went full-time window cleaning. After about three or four years I was hammered with a double tax bill. Apparantly, the idea is to pay your tax in advance, and so, at some point you have to get in front. No warning was given to me, and no option to post-pone.