NB To remove the ring, you need to disconnect the head from the body and turn the ring to the off position as there is a groove for the magnet to slide through. Then slide off with a little force, the magnet might run on the o-rings.
It looks like this :-
On one side there is a small hole to contain a spring and ball bearing ( that sits on top).
On the opposite side of the head is a groove for the magnet.
There were signs of rust around the magnet and the hole so maybe the spring / ball bearing are rusting...
Reassembled the whole thing quite easily. Might take it apart again to oil it.
Update after 30 odd-dives :
The ring now has completely free movement, no notch. The spring has completely rusted up and fell to pieces when I removed it. The ball bearing was fine.
Its too loose underwater like this as it keeps turning itself off, so to provide some resistance to the ring movement, I added a piece of inner tube which seems to work...