For another thread I did stumble across an estimate of tax avoidance - put at £25bn a year. See if I can find it again.
EDIT: Not found my original link but this was quite interesting (and will get a few people going I'm sure!

) But afore they do - yeah, newspaper sources!
"Tax avoidance & fraud
When it comes to swindling, "dole cheats" aren't the biggest drain on the UK economy:
Estimated annual cost (£ billions):
• Corporate tax avoidance: 85
• Business fraud: 14
• Government fraud in Whitehall: 5
• Tobacco smuggling: 3.5
• VAT fraud on mobile phones: 2.5
• Total welfare fraud: 2
• Jobseekers Allowance fraud: 0.19
• Bulldozer smuggling: 0.15
(Sources, respectively: Guardian, 12/4/02; BBC Radio 4, 'Today', 23/8/01; BBC Radio 4 News, 1996; Guardian 17/12/99; BBC Radio 4, 'Today', 3/7/03; DWP, 2003; The Informal Economy, by Lord Grabiner, March 2000; Guardian, 25/8/01)"
Bulldozer smuggling?!
http://www.anxietyculture.com/stats.htm#tax
Some other interesting if debatable - I'm saying that just not to be shot down! - points on that site.