Last answer not strictly true - these mowers are made by an outfit called Global Garden Products, that owns the Mountfield and Stiga brands among others. This means that a drive belt for a 4-wheel Mountfield will fit, which will be much easier to obtain.
Failing that, a call to GGP (based in Plymouth I beleive) will get you the name of a local dealer.
phil_saunders@bigfoot.com
Phil Saunders
August 2008