following yesterday's thread asking advice, I can report job done. Thanks to everyone for their advice.
But it was not all plain sailing....
Mole grips featured in virtually all advice. I had tried various combinations of mole grip yesterday but the pin refused to budge with pulling or even trying to exert a turning motion. And with only about 3mm to grip it was awkward.There is no room to get mole grips between the action flanges so I had also tried needle nose pliers clamped with mole grips to exert a turning motion and even tapped this with a hammer in the hope of pulling the stubborn pin from one side to another. All to no avail.
Until this morning. I figured overnight that a pull on mole grips was quite soft and that a 'tap' on a mole grip clamped on the protruding end might 'shock' the pin free.
It worked ... after a couple of attempts the pin shifted out until the E clip groove was flush with the outer edge. Then the end of the pin broke off, flush with the outer edge.
MMMMM ... time for a coffee and a think. The pin had freed from the majority of whatever was sticking it. It had to come out so it was 'go for broke'. I considered various destructive ways of removing it but decided to use the needle nose pliers and mole grip combination on the pin shaft between the action flanges. It worked ... little by little the pin was moved free. The end nearest the action had a light rust coating, but surprising came quite clean when wiped.

But job done.
Next job... get a new pin. I reckoned that spares maybe available but maybe not ... and would be very expensive if available. So a length of hardened steel was taken from a 4mm steel drill and an E clip groove cut with a dremel. No problem.
The rest of the disassembly was pretty straightforward but virtually all bolt threads were rusted in to some degree which meant some nail biting moments as I turned them ... but all was OK. I am guessing that there had never been any lubrication on many surfaces during initial assembly. There was even light rust on the washer for the end cap bolt. Maybe the previous user had been out in the rain and not wiped the action over afterwards, or maybe the rifle had been stored in a damp place. An interesting observation was that the side lever pivot hinge bolt had been secured in the action by 'peening' over a bit of the thread on the action. Why I do not know. Maybe it was a curious 'anti-tamper' by the person who built the rifle. It was a minor job to free the bolt though.
But all is well now ... fine wire wool and oil has removed virtually all traces of rust and the rifle looks much much better now. I also have the satisfaction of knowing all surfaces are clean, oiled and will come apart easily if needed. I did not touch the piston or breech seals as the rifle was performing well anyway ... approx 6.1 ft lbs.

So ... Batfink now has another target rifle to play with. The complete strip and rebuild took about 5 hours, a job I thought would take me 2 hours maximum based on my previous experience of a 300S. But that had no sticky bits.
Now, what will she get next for me to play with?
Cheers, Phil