Back again ... went back out to the garage and had another go. Ah ha. By the cam there is a circlip that may need turning to the correct position to lever out with a small screwdriver. Also, lifting the brass plate out carefully, there were the springs and tumblers. If you had no key in the lock, they would also hinder it's removal.
What I wanted to do, was to put my lock mechanism into another excellent housing I had that still looked like new, but for which I had no key for.
All done now and can be installed onto my project bike.
If pulling one apart, best to get a sheet of white paper to lay things out on as removed and take note of the order the tumblers were in. If you do get them mixed up, they can be sorted by having the lock removed from the housing, insert the key, and then try different tumblers ensuring they sit down onto the key but remain floush with the top rounded edge of the lock itself.
Cheers ... Phil in NZ