Thanks for your inputs guys. It would appear that the deposits is a red herring and I can discontinue that route.

I have come across many Mondeos with mis-fires before which been tracked down to the usual plugs, leads, coil or valve stick problems. Unfortunately it never manifests any problems when I attend and the misfire is very random. I intend to put this one on the back-burner until it gets worse or the EML illuminates. I will not advocate throwing parts at it until I have something positive to go on.

My post was specifically about the plug deposits and that has been answered for which I thank you all for your kind interest and input.

Regards

Pete M