Now sorted. It was actually cheaper to buy the strings through Thomann than through Warwick (as suggested above).
I also managed to get a 4 string set of Rotos Tapewound for £23 from the eBay seller who did, then didn't, have the 5 string sets, hence the second low B. Like buses!!
I've found that there are a couple of companies possibly trying to "muscle in" on the tapewound market - Warwick and Fender (who have introduced a .58 set) at incredibly reasonable prices compared to La Bella. It may be worth stocking up for what will probably be "introductory prices".