Well it's out - but not with out damage to teh nylon piston which gouged itself on some burrs I made on the cocking slot hammering the b'stard thing out.
I ran out of patience I think and got a bit brutal which is never ideal. The gouges are on the underside of the seal so in a pinch I guess it could be re-used.

Alternatively I may need to add an alloy piston to my list. Curiously the piston was held in place by two short pins and not one long one - did it break maybe

Thanks for the help and encouragement so far. Why did I think this would be simpler ??
Cheers James