Give these guys a call.
Alpha Batteries
I use this when there's no mains, and top it back up between consecutive race-days at the B&B/hotel - sneak it in, in a jute canvas tool bag. Discreet and effective!
Quick tip - your local back street or maybe even main dealer garage have to pay to dispose of old batteries - they should give you them free, if you just ask. Plus there is a way to repair the damage of over-discharge on car batteries, but you need to be a bit of a chemist.... if anyone is interested, I'll post the detail, but it can be found thru Google (refurbish car batteries).
Average car battery can delivery 30-50Ah without major issue, but that's from a brand new one. That's 6-10 charges (at 5A) before damage occurs. That's why some people find that they can use a car battery without issue, but a deep-cycle leisure battery has more usable power and doesn't have the same issue.
The extra £30 is well worth it, IMO.