Thanks Mark. For the record I took the piston out and gently massaged the rings and now they move a lot more freely, cleaned what I could in the ring grooves. Re-assembled the whole thing and no cigar - did not want to start. If anything it feels as though there is less compression than when I started. Now I should say that the large o-ring underneath the starter mechanism is perished but I put it back in with a bit of grease, not sure how much this affects the compression. So it would appear that the rings are faulty but I've rung around and there is a mower shop that might have some in stock and is open.
I do have another engine for the powertorque series, very similar and from what I can see it has a cleaner cylinder bore but I couldn't get the crank pin off this one. A little more rusty than the Corsair but may have a go at swapping the engine over.
