Hi Iain,
if you have upgraded Dragon2000 to the latest version and then changed your VAT rate to 15% after the update (not before) then the system should have changed the vat rate of your 'for sale' vehicles to 15% for you.  Please see here for further information:
http://www.dragon2000.co.uk/news.htm?id=23

If it has not done this then you can change the sales VAT rate by going to the supplier screen and changing the sales default group to another group and then back to your desired group which will cause the system to pick up the new VAT rate for code 1.

With regards to the support lines we apologise for this but as I'm sure you can appreciate it's quite a busy day!  If this doesn't help, please let us know and we can call you.