In message m, jkn
writes
I've opened the thing up and all seems well - the brushes seem OK, the
cable connections are tight, the commutator seems OK, etc.. I've seen
that Dremel do a load of spares and I wonder if anyone has experience
of what might have gone wrong. My current thoughts are maybe the speed
control unit, but given my recent poor experience of Bench Grinder
fault diagnosis (and that I guess this is the most expensive part to
replace) I'm happy to learn of common problems with these units.

Have a good look at the switch or variable speed circuitry. They are
annoyingly prone to causing problems when dust gets into the contacts.
Not so great for a tool that makes dust as part of it's operation.

--
Clive Mitchell
http://www.bigclive.com