One of my LNB feeds is working fine, the other says "bad or no signal". Anyone any idea as to what I can do in this situation? Internal wiring seems fine. Presumably dish is ok as one feed works fine.

Try swapping the leads at the LNB. If the fault is still the same, it's the wiring. If it changes to so the duff feed is now working, I guess it either means the LNB is faulty or dish is possibly a bit out of adjustment.