S1s were supplied with 085 batteries when new. The 097 was a popular upgrade when people replaced their batteries. Either will be fine for a car having regular use but if you are a less regular user then plump for the 097.

The only difficulty these days is locating square post/bolt thru batteries (try your local factors as Halfords are no use). The popular mod now is to replace the battery cables to ones that fit the current round post batteries - You can get adapters but these often interfere with the fit of battery covers etc.

Cheers, John
RSOC S1 Registrar