Where is the battery located? If it's behind the passenger seat, just in front of the rear axle, then I believe the usual fitment size is an 038. I find the Halfords batteries are fine, I replace them every 5 years or so.
If the battery is located under the bonnet then you've got a bit more flexibility regarding size and capacity.
Whatever type of battery you go for, it's a good idea to keep it hooked up to a battery conditioner if you don't use it every week.
