Anyway I went to disassemble it today and found the screw under the jaws had become loose, loose enough that the jaws closed onto its head. The fix couldn't be simpler, just tighten it but be aware that chuck screws in my experience always have left hand threads, so tightening them is an anti-clockwise operation.
Here the steam train has just stopped as the Spirit of Queensland draws to a halt.
I just used a bit of 12mm copper pipe and flared the end. I drilled a hole in the lowest part of the floor judging by where a pool of water from cleaning remained, yes cleaning as I would never splash.
I put some contact cement on it to hopefully act as a seal.