I found the codes and I discovered that they were the same as codes given to me over the phone when I had problems with my Sky+ box.
To access the codes, you go into the Services menu, press 4 for System Set-up then 0, 1 and select.
Option 4 is RF outlets and you need to make sure that RF outlet power supply is set to on.
I tried all this and, unfortunately, it's still not working. I think the cabling must not be strong enough, although it did work for a while when it was initially installed.
Oh well. I guess it's back to finding an engineer who can check the cabling. Does anyone have any suggestions?
Regards
Homer