Well, that didn't turn out very well !!!
I set off this morning, the engine pulling strongly, Just slowing down for some traffic lights about a mile away, and heard a rather shocking, whizzing sound, but the engine was still running, I snatched the cut-off switch, and pulled myself together.
I removed a tappet inspection cover, left the cut-off switch turned off, and touched the starter button, it turned over ok and the rocker arm went up and down.
I really did not like that sound, so I did not attempt to start it, without investigation. so, my friend recovered it in his trusty van.
I have stripped it to investigate, the camchain tensioner has broken, there is a chain scar on the cover, and the camshaft is about 30 degrees out, I have not looked any further, I am worried about the valves being damaged etc.
I think I need to let things sink in, before I decide my next move.