It might be worth just hanging on a day or two, to make sure it's not a signal strength issue, which could be rectified, Clive. The symptoms you describe are exactly that of my Freeview boxes, if the signal is compromised for any reason, but this is usually only temporary. Maybe it's just atmospherics, or could something have caused your dish to be misaligned?
I was under the impression that Sky boxes came with the contract, in which case, you should be able to contact Sky for a replacement, but I could well be wrong on that, as I've never had dealings with Sky. Obviously, if you were to contact them, they would try to sell you something else as well!