On Dec 8, 2:42*pm, "JoeSpareBedroom"
wrote:
OK - so now that we've established that there's a broken spark plug
(assuming the mechanic's telling the truth), what are the various levels of
horror which could result from further diagnosis?

Worst case is something like:

The timing belt/chain is worn and skipped a tooth (just fixing this
could be a few hundred).
This caused the pistons to hit the valves, damaging the valves, a
spark plug, head, pistons and possibly even rods.
Broken bits of valve and/or spark plug gouged the cylinder walls.

Or, you could just have a bit of broken spark plug bounding around in
there and running it for a few seconds with that plug out will clear
it out.

Somewhere in between those two...