On my remote rig V1 boxes I now use the *front* com port, the rear ones as suggested by Mike don't work for me (the FT847 requires a serial pass through so perhaps that is the issue?). I had tried lots of different things including permutations of software settings and hardware configs, but on a whim I swapped around serial port pins 2 and 3 at the RRC control end in case some pins on the serial port pass through were directional. Voila - everything was OK! I went and repeated my back to back CAT cable test (I.e. without the RRC in line) and nothing was working but it did before the pin swap. So, was my serial back to back adapter actually a null cable which had caused me all this pain? When I did a continuity test, it was straight through, so that wasn't the issue! I have to say that I don't really understand, and it isn't so easy to test continuity further as I am using a custom cable from a microham MKII. But it is finally working, so I am delighted.
Next job is to install the kit at the operating desk and make sure everything works over my LAN.