Update.
I was very much mistaken when I said that only one brush had worn down. When I eventually got them out I found all 4 were almost down to the limit. I had been struggling to remove the old brushes when I realized that they came as a complete module, holder spring & brush, all preset. This made the replacement simple once I had found the modules on line, total cost £10.
I have just had a quick run & it appears to be O.K. but time will tell.
The main reason for this update is to thank you chaps for your advice, but also to point out that the brushes in this machine do not have a long life. I have had this machine for just over 3 years & am only a hobby turner. I would advise any one who has had one of these machines for a few years to check the brush gear, as worn out brushes can cause serious damage. I think I was lucky.
Just for reference, a new brush is 28mm long.
Jonas