Without going out to the workshop to measure it, it's about 7" or 8"
diameter. The jaws are worn  and when small diameter work is fitted it is a
off centre by ~0.020". Looking at the jaws I can see where they are flogged
out a little, particulary the outermost 10mm or so.

I occaisionally stick a drill bit or endmill cutter in the chuck and clamp
some work in a vertical mounted vice on the cross slide. I dont know if
gripping the hardened tools has contributed to the jaw wear.

What's the best way to clean them up. The chuck jaws allow external/outside
edge clamping - can I wind the jaws out and fit a ring and then run a boring
bar across the inside clamping surfaces of the jaws as someone here
mentioned a week or so back?

Cheers.