Eventually settled on a U1-R battery as the dimensions most closely matched the original. It is slightly smaller though and lacks the flange around the base, so I'll need to make up some spacers to secure it properly.
This size of battery is commonly used in lawn tractors and small agricultural equipment, so has the advantage of being cheaply and readily available.
They're also a few kg lighter than most other "normal" car batteries seem to be, which given we only have 20bhp to play with has to be a good thing.
