Thanks for the explanation, that all makes perfect sense to me.
Hence, presumably, why engines built with "top hat" liners supposedly cure the problem for once and all?
Examination of spark plugs this morning showed all same colour and no sign of bleaching or washing, so dealer has now started to pull the top end apart. While they do that, I will be quietly praying over the weekend to the cylinder head gasket god (in case one exists) that it will turn out to be nothing worse than a blown head gasket!!
More news early next week, so fingers crossed!
Simon