Poor picture over RF can be down to either the signal or the cables.
Try the RF cable into your main TV and see if the picture is any good there, that will at least tell you the picture is ok when it leaves the box.
If the picture is good to the main TV over RF then you have a cabling issue.
If the picture is not good to the main TV then the RF output channel needs to be changed.
A poor RF signal may be down to a channel clash, so something broadcasting on a near frequency that is causing an issue on your RF Output Channel.
Choose another RF channel as output from the Sky STB.
This is trial and error as you will have to keep changing the output channel and re-tuning the TV to see if the picture is any better.
You set the ouput RF channel in the installers menu that depending on which epg you are running is slightly different to get to.
Old Epg:
Serices on remote
401 [select]
Scroll down to RF Outlets
New Epg:
Serivces on remote
Scroll to the right to Settings
001 [select]
Scroll to RF Out