Thank you, Norm and Twok: I had an M6 that did the same thing and have several friends who have experienced it with M4s. The only mystery remains how Leica UK's service manager could have failed to have no clue as to what the problem might have been! This was a few years back, when I took camera and examples of the partly fogged pictures along to a local dealer's Leica day; I hope they've woken up to it now!