Thanks to everyone for your replies, really helpful, including "you shouldn't have drove"! Sometimes we need the bleedin' obvious re-inforced in plain language to stop us being tw@ts at a future date.
Tom, thanks for sharing your experience - a £ 50 fine and a "don't do it again, you knob" would be a fair cop, I'll take that on the chin if that's what they hand me down. Sorry to hear about the business miles, that sucks. I used to do 10-15k a year and it would have gutted me to lose money on it. Don't even get me started on this government and new indirect taxes. We get shafted in this country but we often don't realise just how much.