Well I may have solved this, I'll know for sure tomorrow when I drive the car some more.
I noticed that the #4 spark plug lead was quite loose, and pulled off the plug quite easily. I looked more closely and the end of the plug was horrible and brown, as was the inside of the lead where it connects to the plug.
The #4 plug is consistent with where I was hearing the squealing noise from.
This evening I replaced the spark plugs and put on a 2nd hand set of spark plug leads. What concerns me is the condition of the plug:
there is some sort of build-up on the plug. Any idea what could cause this?
It is also present to some degree on the other plugs, and there was a little oil on plugs #1 and #3 as well.