First thing to do is check all connections to your Sky Box i.e Aerial input and the Scart Lead as sometimes you can accidentally 'dislodge', causing bad connections.
Have you got easy access to the Dish itself as sometimes you can get water entering the LNB/Aerial connection and cabling causing Earthing problems.......
The dish is fixed to a side wall and I don't feel confident of scaling a ladder myself but will get one of the grandsons to look for possible water entry.
I will recheck the connections to the box.
Thanks for your input.
Get the connections on the Sky Dish itself Checked first just incase water has somehow got into cabling. Happened to me many years ago and LNB had to be replaced. If all appears OK go into Sky System setup and check various settings...
just a quick idea, make sure you've not got the volume on the sky box turned down low and, consequently, need the TV volume turned up higher than normal as this would introduce hiss.