I'm sure lots of you already know this but for those who don't and are still paying through the nose for RedTops, there is an alternative. mdsbattery.co.uk supply lots of small size batteries suitable for racing cars. The range originally designed for rather slower transport, the mobability scooter, work very well as race car batteries. They are very good value (possibly even less than half the cost of RedTops) and the people at mds are very happy to talk on the 'phone and find the correct product for your needs.
I have no association with the company...