[Technologist]

The 27.5 W services are a basically the whole two DTT muxes  Not individual services 
which at a main site  Is the third path ….. used when both telco links fail.
So the BBC Just feed a duplicate of the mux to the satellite for the failed site .
(And hope no other region has failed) 

For the Bilsdale relays it just needed to be modulated  and applied to the transmitter …. 

With clever maths it is possible to remove a service from a statmuxed
Stream and substitute another service which  itself can be in a satmuxed stream.
All without decoding  the substituting service …..
I demonstrated this to the BBC on 6/7/2005 ….You could not really see the join !

A use case may be a north Devon  relay  …it can receive Wenvoe perfectly
So drop in BBC one SW from DSAT …

But it was expensive …. And not thought reliable
So you will find a number of line fed relays on the north Devon coast