The mechanic told me that he had noticed deposits on the sparkplug when he removed it during the service. He also said that if you shine a light into the cylinder you can see some staining, presumably from the coolant fluid.
I haven't used the car much in the past year or so. It has been running fine & I haven't had to top-up the coolant.
It did cross my mind that the issue was cause by over tightening the plug, possibly on a previous service by the same dealer. When I collected the car, the mechanic at the dealer also advised that this was a potential cause.
Bottom line is I can't prove anything, so I have no alternative but to get it fixed (I'm too honest & won't trade it in for something else & pass on the problem). So if anyone can recommend somewhere I can get it fixed it would be greatly appreciated.