Paul,
That's a great story.
In all the times that I have repaired TV systems like the type that you mentioned, the problem has been caused by the homeowner, who has used the original coax (normally single-screened R-206) or twin lead (ribbon) cable and fitted the highest gain amp in the land, to the system, in the hope that there will be MORE signal at the other end of the cable.
With respect to Output channels on Decoders, the Sky UHF Decoders over here, have a pair of 10 position (0-9) Rotary switches on the rear panel and these are used to set the Output frequency.
On the Digital decoders, the presets can only be accessed by inputting an installer code.
One thing I do know about Digital TV recievers, is that if there is a small fault in the input signal quality, you don't get a picture at all, as opposed to the snowy picture on an Analouge TV.
How do the Minimum Signal levels required for a Digital TV compare with that of an Analouge TV?.
Were F-connector type plugs and splitters used in this installation?. [Linked Image]