Note they changed the rules again after all the compliants about the 2001-2006 thing.....
http://www.direct.gov.uk/en/Motoring...icle/DG_172916
Hence pre-2006 still never pay the 225g/km+ rates.
The 2001 date is still relevant too as before that the flat rates for above 1500cc etc. apply.
They did try and make the post 2006 rates apply to cars all the way back to 2001 - but as a retrospective tax this was very unpopular and thankfully the recession seems to have killed off the idea (for now).