Thanks Bogs.
I forgot to add that some clearance between the OD of the tube and the bore of the chuck is very important. If the fit is too close the tube won't be able to float to balance out the preload on all three jaws. If the slots are all the same width it isn't really a problem. I think that the OD on the three I have made is about 1mm smaller than the bore of the chuck.
Apologies for the size of the photo - I'll upload a smaller one next time.
Phil.