the chain came off the sprocket while the bike was ticking over,thankfully.
the head was removed,4 very light indentations on piston crown.
valves inspected & tested,all 4 were fine.
the camchain showed no real signs of damage apart from 1 or 2 very slight tight spots & 30 odd k worth of stretch.
the tensioner had popped but then it would anyway as soon as the chain went slack.
i was of the opinion that the sprocket was out of true due to the dishing & that the chain simply ran off the side of the sprocket,there didnt appear to be enough damage on the chain for it to have jammed up against the sprocket.
thats just my opinion,i could well be wrong.
anyway,one new camchain,a gasket set,a sprocket courtesy of mark ( thank you ) & a lot of very helpful information,courtesy of this brilliant forum & its members ( thank you too ) the savage lives to ride again.
it is on the button,totally leak free & sounding better than it has for at least 15yrs os so.

i agree lancer,ideally i would have but it is owned by a 78 year old pensioner   who could couldnt afford to have it repaired.
i did all the work free of charge & i supplied all all the parts at cost price.
he had change out of £150 & that included new plug,oil & filter.
alternatively it would have sat at the back of his shed forever as it means too much to him to get rid of it.
certainly the lesser of two evils,im sure that you will agree lol

taff wrote on 08/10/18 at 17:05:14:
--------------------------------

I hear what you are saying, but playing devil's advocate...

Truly evil would be if this 78-year-old man or any other person were riding along at high speed and the engine grenades and locks up.  Then before they realize what's happening they get slammed to the pavement and they get run over by a truck.  

Unlikely, but surely not the lesser of all evils.