When I first built my Sumo I went to Halfords and picked a few belts till I got one the right size - pure trial and error. Now….
It turns out that although fan belts have proprietary type numbers, they're mostly labelled on the belt itself with the width and length - so if you know that, it doesn't matter which make you buy. A Sumo with the Pilgrim modded alternator/water pump setup needs a 10x1063 or 10x1075 belt.
The Pilgrim way of tensioning the belt is very unsatisfactory, with a sliding slot system which runs in the wrong direction, making it very difficult to tension. Tinka, of the Cobra Club forum, came up with a much better way to do the job, which I've modded very slightly. From McGill Motorsport on eBay, buy -
1 x Turnbuckle M8 Link, Adjustable from 190mm to 220mm
1x M8 x1.25mm Male Rod End/Rose Joint Left Hand Thread 8mm BRONZE LINER (MMSAKL8B)
1x M8 x1.25mm Male Rod End/Rose Joint Right Hand Thread 8mm BRONZE LINER (MMSAK8B)
M8 nut to lock position.
These go in place of the Pilgrim slotted strip, and work much much better, in fact you can set the tension with just your fingers. Actually attaching the thing in the first place is quite tricky, as you have to get the length right before you can put in the screws. Someone holding the top end on a piece of string whilst you attach the bottom end will work.