This sounds like a problem that has been reported several times on here recently. It involves channels that use Sky's AdSmart system (eg. Channel 5, Sky's own channels, Comedy Central, Nat Geo, Discovery, Quest).
Other channels like BBC, ITV, Channel 4 and the U& channels are unaffected.
Unfortunately there is no solution other than change channel then change back. While there have been several reports of this problem recently I don't think there have been enough for Sky to recognise it as a widespread problem.
