Thank you, Gentlemen, for your replies.
I've tried the re-booting option, in particular the version as laid down by
Green Granite.
I thought we'd struck gold as the TV behaved itself faultlessly for twenty-five minutes and then............ well, you know the rest!
It isn't a particularly serious problem, but it is very irritating. I have a feeling it must be the LNB. I think those supplied with the Sky mini-dish are of a non-standard variety and care is required in purchasing a replacement.
Retires to scratch one's head.
Thanks chaps,
N o t a