Alex have you wired up your Two 7.2v Batteries in Series mate? This will give you 14.4v which will make it a bit faster than your 12v battery, though not too much difference, just you will save alot of weight.
Have a quick look here mate - http://www.stupoo2000.pwp.blueyonder.co.uk/teamkat/batteries.htmhttp://www.stupoo2000.pwp.blueyonder.../batteries.htm
You want to wire your Two 7.2v Battreies the same as the first diagram on that page.

Mr Stu