Just finaly got round to spending some time on this, it was definatly caused by a faulty DiSEqC switch. I did some process of elimination and bizarely if i covered the C -Band LNB the signal dropped, if i covered the KA this also happened. (The C is port 2 and the KA port 3 on my switch)
Out of interest i checked the other two sonnections (1 which is my main KU and 4 which goes to a smaller dish round the side of my house) the smaller dish didnt seem to be working at all so i checked all the cable runs & LNB. I found a big kink in the cable where it goes near our gate, looks like theres a short here and its blow or partially damaged the switch. I'm also now getting the KU signals form my main feed on port 4 on this switch even though its conencted to port 1 (they are coming in on both) so new switch is the way to go, luckly not much expense.