Any mainstream Raleigh dealer should be able to help, since in the UK it's really only Raleigh that still use TranzX drive systems on some of their e-bikes, and they would (I assume) still maintain a relationship/account with TranzX.
There are some ebay sellers that sell compatible batteries.
Alternatively, you could likely have it re-celled: ebikebatteries advertise here and have a good reputation with many members here for re-celling.