Drive belts are a cheap commodity item.
Assuming it's a trapezium (blunted wedge) shaped belt, there are a few standard sizes for the cross section and lengths increase in again a fairly standard set of sizes.
Wind the belt all the way round by hand and look see if the details are printed on the outside of the belt.
Assuming they are, you can buy online from bearingboys, simplybearings, engineersmate, and a host of other sellers.
If Axminster want too much for their belt (and sometimes saws use 2 or 3 belts in parallel) you will have to measure it. The width of the wide, non rubbing, outer face will translate into a size like an SPA (10mm) or SPZ (13mm) cross section.
This has more than enough info:
https://www.powertransmission.com/articles/993-guide-to-v-belt-selection-and-replacement
The length you just have to measure. If the belt is wrecked, just cut it and stretch it out to measure.
Give a thought to whether the saw is set to the shortest or longest adjusting position when you measure the existing belt and pick the nearest standard length replacement.
