Try one of these if you leave it outside permanently. May get enough good weather for it to become useful

http://www.maplin.co.uk/Module.aspx?...=47902&T=39442

If not, trickle-charger: http://www.argos.co.uk/static/Produc...2Bchargers.htm

Just to add: I went through 3 batteries in 1 yr before working through this with a friend and removing all sources of drain to find out it was the stereo. So called "electrical experts" spent time and my money before just saying they couldn't find anything wrong with the car.
Buy a new battery by all means, but don't let it stand until it goes flat otherwise you will have ruined a good battery for no reason.

If it's an S1: stick some pics up