Well, after some research, I decided to go down the electrolysis route. I filled a plastic tub with water and soda crystals, made a couple of anodes and immersed the complete gun, connecting the lock to the negative side of a 12v battery charger and linked the anodes to the positive side. Bubble, fizz etc....
I re filled the tub 4 times due to the water getting very mucky.
The barrel removed ok, the tang screw was missing.
Appearing on the lock and barrel, were 'E. BAKER & SON, which was nice...usual pair of proof marks, plus a GR with Crown.
The 2 lock screws turned, the front one coming undone, but the rear shearing in half as it was so corroded; the remnant of the screw came out the back of the lock with a little heat. I then soaked the lock for 3 days in WD40.
Heating the lock, allowed me to remove the ****, frizzen, frizzen spring, with no issues. **** jaws came apart, also with a little heat.
The lock inside looked grim, to put it mildly! The mainspring was broken, as was the sear spring. Surprisingly, the screws in the lock all came undone, again with heat.
I found and modified a mainspring and a sear spring (among all those bits I've been stashing away over the years). The lock is now working, and shows little wear.
The stock was very soft, so I drowned it in Ronseal wood hardener, which has improved the strength of the wood. The pins holding the trigger guard, trigger and barrel came out , again with no problems.
The cracks and holes in the stock, I filled with pure Beeswax, which I have a very large block of, from my Grandparents farm. I break a bit off, warm it in my hands, roughly squash it into place and then shape/melt with the solder attachment on my little Dremel gas torch. The wax sets very hard, but has a bit of 'give' which works well. (It also allows anyone better than me at wood repair to undo any of my filling, in the future).
I finally made a piece to replace the missing bit of trigger guard, again the screw came out the wood with no problem
I'm quite pleased with the result.