Was smoking for a while, then started to tap and noticed a power loss. tapping seems to be coming from right hand side.
When looking at the LH spark plug found heavy black deposits (Haynes manual puts it between carbon and oil fouling)
Had low oil (200ml) but took the rocker cover off, and turned the engine over (to check for obvious issues with the valves) and noted oil was still being pumped over the cams.
looks as though the next step is to take head off and inspect piston rings and valve oil seals, any thoughts?