sgmilne and 13977,
It sounds like you're in the same boat. You may have some luck trying Digital River/Matty but if not you could try the UKBA. It took me some time to find the link but have a read of this - http://www.hmrc.gov.uk/customs/post/paying-tax.htm#2.
I nearly had to go down this route when I ordered some stuff from Telltale Games but they refunded me immediately when I emailed them about the customs form being incorrect. Now that was customer service!
Good luck and let us know how you get on!