Thanks for your advice chaps. I avoid buying from Amazon for most things as there have been tales of counterfeit strings for years. I usually check prices on Strings Direct, but usually go with a couple of sellers I've used on eBay in the past.
I ended up going with Gear4Music in the end, and decided to try the Warwick Reds as suggested by @Lozz196 rather than the RotoBass or Fender as originally intended. I've bought two sets, one of which will go on my Bitsa P. I can't remember what went on there last, possibly GE Bass Signature Strings for a Fiver on eBay, and the four months I've spent Shielding from Covid has killed them.