Initially, I thought that It had just worked loose as the plug leads were less than a year old.
On closer inspection, I found that the retaining clip in the plug lead had come off and was stuck on the plug. When checking the remainder of the leads, I found that all of then had the same problem. I also noticed that the rubber seals on the leads had hardened and distorted to the point of being totally useless,

This was a kit sourced form Goldwagen
I decided to stop messing about and fitted a set of Bougicords from Midas. (They had to be specially ordered, but it was worth the wait)
