Ah. Thanks for that info unfazed.
I think it's plain worn out. It's done 121,000 miles and the when holding the bike on a steady throttle, throttle just open (as you would when doing 85 on the motorway) I can feel a slight hesitation. It's very slight, but just recently it has started occasionally flicking the tacho needle and pointing at 3000rpm.
I tested the TPS according to the manual and it registers 6k ohms at rest, then just slightly open 10 or 11k, then down to 5,4,3,1 which I presume is the track on the potentiometer being worn out. I could easily be proved wrong by someone who knows what they're on about. This is just my guess.
I've found a tragic looking pair of T Max carbs on flea bay for 25 quid, TPS intact, so that's good. If only he knew they had a 200 quid part attached to them!