Originally Posted by SafeChav
It may have just corroded, and then the compression has blown the rot on the plug out.
The plug is going to rust away well before that head does
lol
Penetrating oil on its own should see you through, and as i say use a nice long breaker bar or ratchet to undo them so you have more effort against them
Not necessarily a good idea to put a breaker bar on them, if they are properly seized that's a sure fire way to snap them off at the taper. But there's little way round it if they are welded in there.
I'll favour the softly softly appraoch I reckon, after a liberal soaking with oil. Is it best to try it on a hot, warm or cold engine you think?