Local motor factors are always cheap if money's tight. If you look in Halfords guides you can get the code for the motor factors

Or you could buy a Halfords battery, I think they do 2 types, a normal one and a new cadmian (sp?, or something) battery.
Mine cost under £30 from the motor factors, it's a micra battery :?
I assume the Sunny takes a UK (big) battery :?: