I own an old German W Fortress One and also used an FNA fiver for a few years. I'm ambivalent to them, frankly, and I disagree that "nothing else sounds like them" as I have a large stable of basses and have a number of basses that can replicate the Warwick sound.
Warwicks are both hideously over-expensive and massively prone to depreciation. The OP is British, clearly, by dint of the currency he mentions - he'd do far better to contact Alan at ACG in Scotland and enquire about one of his "ACG Graft" line of basses. The ACGs are handmade individual basses, built with love and care by just one guy (who happens to be an incredibly skilled luthier), while the Warwicks are made on an impersonal mass-production line. Amazingly, the ACG Graft, both 4 and 5 strings, is considerably cheaper than buying a Warwick in the UK.
Pricing of ACG grafts is here: Invalid Link Removed
while Grafts soon to be available are shown here:
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Hope this helps.