Picture break-up is nearly always a sign of poor signal strength, so you need to check that everything is OK with your dish and aerial. The power of the transmitter on an Astra satellite is only about the same as that of an electric kettle, and its 22,000 miles away, so your dish needs to be perfectly aligned with it or you won't get good reception. Perhaps strong winds have knocked your dish (or its LNB) out of alignment or maybe a tree has grown and is now blocking the signal path?
The only other likely cause of your problem is interference from another electrical device. If your Sky box is very close to your TV (or to a computer, printer or other electronic device), try moving it.