Have you done a Gooogle search for - say - "Challenger tool spares"?
that said, you'll probably not be able to find any meanginful spares.
Pile 'em high and sell 'em cheap Chinese tat, sold through Argos, I believe, with effectively zero support.
Its the same with "Qualcast" - a once proud name bought up and put on unbranded imports in the vague hope to command a higher price and better sales with what people remember as a respectable brand.
You MAY be able to open up the old battery and replace the cells but in the end it may just be as cost-effective to buy something new with a bit of a warranty.
Or shell out on a brand with proper support.