Simply measure it’s physical size, note the ampere hour capacity and cold cranking amps, which will be shown on the battery label, the type of terminal whether round or flat and then pop along to halfords for an equivalent one at a reasonable price.

Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900