Quick update from me. Ordered my belts on Thursday; 9 belts cut to 3mm width came to under £10 (so about £1 per belt, amazing!). And they arrived Saturday morning less than 48 hours later, absoutely fantastic service from these guys.
Very pleased with the belts. They are slightly stiffer than the ones normally found on touring cars, as they're about 0.2mm thicker overall, but the tooth profile looks spot on and I'm sure they will free up with use.
Here's the website for anyone else wanting custom belts (or just strong, cheap belts; these things definitely won't snap):
http://www.beltingonline.com/3mm-htd-timing-belts-4573
These guys are the saviours of the DIY car builder!