I have a similar problem quite often and have to turn he set off at the set not the remote and then on again and all is well. It does get annoying though if in the middle of a drama or film.
Does your telly turn itself off then for no apparent reason? Check to see if there is an auto power off timer or something similar. Mine did that when I first bought it - the factory default was set at 2 hours. Am still not sure why it would have a facility like that at all...
Use your John Lewis 5 year guarantee. If you didn't use John Lewis, use them in future.
My HDTV yesterday went wrong, when you turn it on the picture is perfect but 3 seconds later the screen goes black but the sound still works.
This happens every time I turn it on.
Is it terminal or is it a simple repair ?