RRU, Unplug both F connectors (the connectors the sat cable plugs into). Then plug them back in. Sky did a poo job of installing my HD box. It's amazing how easily these little connectors come undone.
Channel 4 uses 10714H and 10729V both on the Astra 2D satellite. This satellite is also used for the vast majority of the Freesat channels as well as BBC. As the majority of the channels on this satellite are unencrypted the satellite has a very tight UK footprint. If your dish is even slightly off you'll start to notice picture breakup etc.
If you don't have any luck with Sky, you could try getting a local installer to check your alignment, it will cost about £30. Like another post has said the Sky LNB's have a habit of breaking, especially in cold weather, a replacement dish and LNB is less than £20.
Hope this helps
Ben