Great tips folks, many thanks, and hopefully, it'll help other newbies to the TRX look for decent bikes as well.
My plan was to look for an as original as possible bike rather than one that's been played around with too much.
There was/is a "streetfightered" one on the "local" Gumtree, digi dash & FZS1/6/8 style headlight & loud cans, for £1400, no mention of when the work was done, whether he'd still got the original clocks/fairing/cans etc. When there's decent, standard looking TRX's going for around the same kind of money. Missed out on a blue one at the weekend on eBay, but it's a long way away, and I didn't have the cash at that time.
Budget's around £1400-£1500 for a decent, standard TRX... (or another Hinkley Triple). From the research I've done, at the moment, it seems like the going price for a good one with reasonable miles on it (seems to be around 20k). Loads of dealers in the UK seem to be asking top dollar for their second hand bikes, even with big miles on them. It seems like people are keeping hold of their older bikes for longer because of rising prices/priorities elsewhere (and the fact that dealers are asking more money for their bikes doesn't help the market either, which is the viscous circle really).
The search goes on...