Hi Swanfis:
You said
All three frequencies are "cross coupled" which means that a call from an a/c on one frequency is almost instantaneously transmitted on the other two.
You must have found that it's quite common for an aircraft not to hear the transmissions of another aircraft on the FIR frequencies, but to clearly hear the ground reply.
Given what you have said about cross-coupling, this seems to mean that the aircraft which a pilot doesn't hear must be transmitting on the frequency to which he is listening. The two aircraft are then further apart than line-of-sight, but each is within 'sight' of a transmitter. Am I right?
I'm amazed that there are only nine transmitter sites to cover the whole of the London FIR. When I'm really bored I'll do the maths and prove to myself that it works.
AA.