If it’s of any help to others, the battery is a V625U.

Splitting the cartridge is straightforward but requires a thin, sharp blade. I used a modellers knife. Also very helpful to find the split line under a magnifying glass. Mine now works ok with its new battery.

Replacements cartridges are available but hard to find and expensive, I was quoted £25. The period part number is 6875397.

A later part number, 92GG T22053 AA looks as though it will fit and is more generally available and less expensive.