Thanks Bojan,
I did see your idea a while back while churning through IIS archives
and remember making a note of the Mosfet DC changeover
circuit.
The clutch works well by the way.
I wanted a setup where I could manually focus, but also have
slippage if I accidentally left the lock on the crayford or it
fouled on an end stop.
This seems to do the trick quite well.
Steve