Today the original battery has about an hour's capacity, and the second one is below two hours. So I went along to the big Apple shop in Regent Street and asked prices tonight. And it turns out they cost £89.95 each. And unless I can find replacements a hell of a lot cheaper than that I'm afraid that my days as a Mac user may be somewhat more limited than I'd anticipated.
So if anyone sees anyone selling genuine batteries for a 14" screen iBook G4 cheapish please let me know.