Thanks for the advice.
I don't think there's anything in the sides either, in fact I took the sprag out yesterday and can see nothing wrong with it - it works fine and last year I had both sides off and could find nothing. I even had an endoscope camera in and could see nothing and the top ends (valves, cams etc.) all looked good.
I didn't want to split the engine at the time so put it back and it worked fine all year, but it can't go on shedding that much metal before something gives so I'm going to have to go for it this time or face a potential disaster when something lets go.
As for the shards on the plug, yes they're small filings that have become like stalagmites, held together with magnetism and oil.
Last compression test was last year and there was plenty (228psi vertical & 215psi horizontal), I've not done one this time but may do before pulling it apart although I'll have to refit some parts first.
I will dissect the filter, last years was full of filings so I'm expecting the same, or likely worse this time.
