cant give the numbers but i use the same ones as for a mazda car.
incidentally, you can get the 13mm wide (dont use car spares ones, they wont work for long) drive belts for water pump and alternator from fenner power transmissions - see yellow pages.
Coopers Z819.
Regarding the comment about drive belts, I've had the same Halfords alternator belt on my 2GM for 4 years now. How long is "not long"?
Dan