Sorry to resurrect an old thread but if anyone, like me, has much the same problem, this may help:
1. If the battery seems completely dead, it is worth opening it up at the plug end and checking the fuse, which is a standard "euro" style fuse (as fitted to loads of cars) and if it needs replacement, you should be able to get one from Halfords or similar.
2. It's very difficult (I'm trying not to say impossible) to get a straight swap off-the-peg li-ion battery pack that will fit inside the case, because of the way the cells are arranged. There seems to be 30 cells (18650s presumably) arranged in 3 (wired in parallel) blocks of 10. Each block of 10 is arranged in a 3-4-3 "stretched-hexagon" shape, wired in series to give 36-37v. So the cells themselves run length-ways in the case. All of the off-the-peg li-ion packs are assembled so that the cells would be oriented across the case, and since the cells themselves are 65 mm long, the packs are usually around 70 by 40 mm - too big to fit in the case which has an internal diameter of 73 mm.
Writing this now, I'm wondering if the cells in the later "bottle" style packs have the same configuration as the X-6/X-24 pack.
But if you want to renew the original Powabyke battery pack, the only option AFAIK is to take it to a re-cell specialist.
The simpler option to me seems to get a rear rack mounted battery which I've seen for £229 (including rack) for a 10 Ah unit, much more than the 4.4 Ah OEM one. This may be the route I end up going down.