This weekend I've finally had a bit of time to see if I could free up the seized piston, it was left to soak for a couple of days with some oil in the cylinder as I usually do, then I used the flywheel to turn it gently each way a little bit at a time until it was free.
Once the piston was free to move, I could safely remove the cylinder without any further damage.
It looks like this early engine had little or no use.
I think I may have a spare con-rod somewhere, just need to locate it.
Looks like it has the odd sized piston rings, although it shouldn't have according to a service bulletin (last used in 1961 with serial number 006072) as this is serial number 006403, I guess they still had a few left and used them all up.
David