There seem to be two methods, which are dish height dependent.
(1) lower dish. Hit it with a broomstick to dislodge ice and snow.
(2) higher dish. Throw a football at it to dislodge ice and snow.
I have used both methods successfully, although I expect they are frowned upon by Sky.
* I know the picture is of a barbecue. The Sky dish picture was a bit too blurry
