Chaps,
Firstly, thanks for the suggestions & offers. Some more info for you:
The chuck is a 10" 4-jaw independent. I just measured around 0.005" runout in the chuck body, but zero taper across as much of the length as I could read without changing the dial gauge setup, around 3" worth. This is the chuck that was on the lathe when I bought it, so I have no idea what abuse it has suffered in the past, only what abuse it has suffered since I had it (and that's some - this chuck has seen me from never touching a lathe, to now - nearly a year later).
The 2" steel bar I dialed in last night shows 0.002" TIR at the far end, and about 0.018" TIR at the chuck end. I know the bar is straight because it rolls quite cheerfully on my surface plate with no appreciable light gap. I also get the same effect whichever stock I put in the chuck - I have some 2" ali which is the same, some 5/8" tool steel (same) & 5/8" regular steel (same)... I can also sometimes knock it square with the plastic hammer (provided it's not in the jaws too tightly) - inevitably, when one tightens the jaws, you can feel when the stock moves off parallel again.
Dave - it is true I've had the chuck off a few times (it's bolted to a back plate type affair). I did once trap some swarf between chuck & back plate; that was instantly noticable (you could see the chuck wobbling), so I know it's clean back there at the moment. I've not actually removed the jaws to give them a really careful look-see, but I'm pretty sure it's not compacted swarf that's the problem. That said, I'll give 'em a good wire brushing to see if that fixes it.
David D - I had the same thought as John, the jaws are clearly hardened (they mark everything that comes into contact with them, they've even (slightly) marred the ER32 chuck I have which has a hardened body. I guess a carbide tip might touch them, but I'd be worried about chippage, not to mention getting the things accurately set in the first place.
John - thank you for the offer of the surface grind, if the wire brushing fails to have any effect, then I will be in touch for sure. I'm only about an hour's drive from you - if that - and I may well be passing Crewe in the next few days on another errand... I will PM you this afternoon if I may.