Just to add a bit more; in my case mentioned above, as Steptoe has said, it was a 'sudden downpour'; like someone had tipped half the Atlantic on the M6. I wasn't thinking of water going through the oil cooler/radiator, just all of the spray going directly on to the cylinder heads. It even cooled the down-pipes enough for them to show wet! Once out of the rain and a few miles on, it climbed back up to the nor,al 4/5 bars. Never noticed it before, never happened since. I'll watch out the next time I hit such a downpour, which shouldn't be long up here...
Dave.