Thanks. The receiver is the Humax Foxsat HDR, and the satellite dish is 63cm. I have tried reconnecting all the cables. The HDMI cable is the freeby that came with the set. Both times the problem has occurred it has been wet or very misty. And later it works for no apparent reason..When I have the problem, on trying to change channel the picture and sound appear for a mini second, then the screen goes completely green.
I notice also that when the set is programmed to record one channel, leaving the other HDD free to record, several BBC channels are unobtainable, so it's not possible to record say BBC1 and BBC2 together. Obviously I have had a double LNB fitted!
Any advice will be gratefully received, as I can hardly call in the local electrical shop for a piece of kit they don't know.