Westwood 36" cutter drive belt specs?

I have a Westwood T1012 lawn tractor with 36" deck. Am trying to find the spec for a replacement deck belt. Have tried an A70-13x1775 but it rolled over and stripped quite quickly ( 4 lawn cuts x 0.7 acre) I need the generic belt details. it may be A, B, or SPA or SPB and some numbers. Any ideas beyond original part number and a £26 price tag as Marksman Industrial do belts for between £4 - £16. Just need to know which one!