Need 4jaw indep. Chuck for Hobbymat MD65...

I'm looking for a 4 jaw independent chuck for a Hobbymat MD65.
I've seen a Roebuck 85mm chuck at about GBp 120, and the Axminster one
looks similar.
Has anyone any experience of fitting one of these to an MD65? What do I have to do? Which chuck is better? Can I use a 100mm chuck, or will only the 85mm one fit?
Many thanks,
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