Thanks for all the responses so far, however I still don't have this working, for some reason.
At the radio end I have a K3/100 + RRC-1258 MkII.
Originally I had a laptop at the 'control' end (I tried various programs like N4PY Pegasus and K3Span). The laptop was connected to the control-RRC via USB and the Radio Settings/Program Mode was set to NO.4 (Yaesu, Kenwood, Elecraft) on both RRC-1258 units. It worked fine.
I then removed the laptop and USB cable from the control-end and connected a K3/100 to COM2 serial port on the control RRC. I could still hear the audio through my headphones at the control end, however when I pressed and held the Menu on the Control-K3 it came up with TERM and stayed like that - there was no control over the remote radio.
I then changed the Radio Settings/Program Mode to 14. Elecraft K3 twin on both RRC units. The control K3 still didn't do anything and the audio disappeared. The speed was set to 38400 on both K3s and both RRCs (COM0).
I hope that makes sense - any suggestions please? What am I doing wrong?
Thanks John G4IRN.