Took the head off over the weekend and took some measurements. Quite clear that's all been hitting!
Measured 0.63mm on the head gasket with a micrometer.
And used a straight metal ruler and feeler gauges to measure the piston deck height. I got 0.72mm above deck on its highest edge/rock and 0.35mm on the lowest edge/rock.

Going to buy a 1.4mm gasket and listen out for any worn big ends. A lot easier than changing them if they turn out to be fine IMO.

Will let you know if I get the rattle of death this weekend!

Some photos: