Thanks for the replies.

I didn't think a block or pistons could distort easily or so evenly; all 4 pistons seem to have near enough identical amount of movement.

Rings sound more likely to be the culprit...

There seems to be only light scoring in the bores in small areas so it certainly wasn't excessive.

The engine is being stripped down for inspection and to measure clearances Saturday, I was just wondering what people might think at this stage.