I was going to suggest do you have other receivers connected to your home dish as these could be powering the LNB, if so take them off the system. Around here used receivers are about £10 so depends how much messing about you want to do.
BUT I can see the challenge of getting it sorted
Firstly can I ask how many coax feeds you are plugging into the Sky receiver?
With a Sky HD box they have two input terminals on the rear of the box (input 1 and input 2). My first thought would be (if you are only using 1 cable input) to make sure that you are in input 1. It might sound silly but because input two is on the left a lot of people just screw into that one assuming (like a normal person) that it's input 1. Input 2 is used for recording purposes so will not give you a primary TV signal.
Also, if you are only using the 1 input then some Sky HD boxes need to be put into "Single feed mode" before they will operate on just one input feed. If you want to check to see if that helps then a step by step guide can be found here: https://skyepginfo.co.uk/HowToGuides/Single-Feed-Mode.php
If you are using a Twin Cytrac system and have two leads in the back of the Sky box then I would try swapping over the inputs. With a Twin LNB there are actually two independent LNB's in one casing so it is possible that one side of the LNB has gone down.
If none of these work or help you then it's probably best to give me a ring at the office on 01858575928. That way I can get a better idea of what's going on and hopefully fix the problem for you.