Thanks to all of you for responding to my query. You have taken a real interest and offered some good advice. Sorry about the delay in posting again, but I had a number of other urgent jobs to attend to (like repairing my fence after the recent storms in my area). Anyway, The Problem is SOLVED. Hurrah! Today I examined both ceiling roses ('cos that was easier being more accessible)and carefully checked and cleaned the first one and followed this by replacing the second one which had a cracked bayonet fitting (plastic) which broke off when I started to take it down. I thought that was problem solved, but it wasn't that, so I spent over an hour removing books and shifting the bookcase to get at the switch. Those of you who have partners will know that inevitably there was then a bit of a delay while my wife vacuumed where the bookcase had been! I could not believe what happened when I unscrewed the switch and pulled it away: the neutral wire just flew out and hung loose. There was absolutely nil contact between the holding screw and the neutral wire (it must have been like that for over twenty years!!). The circuit was being made by the springiness of the wire pressing against the side of the neutral terminal! I tightened it up and Hey Presto the whole thing works fine. I still do not understand why I have never had even a flicker of a problem when using at least one filament bulb in the circuit, but I'm not complaining! The message seems to be that LEDs are very sensitive to resistance in the circuit and require good connections throughout, unlike filament bulbs. Thanks again to everyone for your suggestions.