Loose spark plug

Bore wash on that cylinder would not be good and raw unburnt petrol into a hot cat could cause major damage to the cat brick...

As above.

If loose, it could well end up being pushed out through the forces associated with compression and power strokes, and this would likely end up stripping some of the threads from the head.

--Rich

I had a loose one but spotted it quickly and nipped it up, seemed ok after that but then the head gasket properly went not many moons later :}

I heard a "clopping" noise in there and as it happened I was in Leicestershire picking up a radiator from Fred22, who diagnosed the issue when I was showing him my "tick of death"... "Oh, there's a new noise, WTF is wrong now...!?"

haha i remember that :} glad you got it sorted out before it blew out of your head, that would have needed a helicoil to sort it out, but anyways im glad it didnt get to that stage :L

The new ones have gone into nicely cleaned threads and tightened up to the proper torque.