It could be the LNB in your dish. Dish out of a alignment. Cable from dish to STB. (Never check the latter with power to the STB. Or, as it is a Grundig there is a well known fault with some of their internal power supplies.
The fix for the last problem above consists of replacing all of the capacitors(about 12) in the box's power supply. You can buy a kit from a company called satcure. If you can solder (it IS a skill) then you could try the kit.
Alternatively I believe that Sky will charge you about £65 for a call out and if the STB is broken replace it with a refurbished box at no extra charge. I think that you should call Sky.
t, July 2007