Right, some kind of progress, but even more puzzles:-
Occaisonally when resetting, one of my units will show the addresses (pipe) as E2 & D2, whilst the other shows both as D2.
If I try transmitting from the E2/D2 then I get nothing. If I try transmitting from the D2/D2 then the E2/D2 responds as you expect and works ping/pomg. If I then change the D2/D2 to 'R' and the E2/D2 to 'T; then it works again! Strange, it will only work that way round AFTER I've had it working the opposite way.
I suspect that if both units show E2/D2 correctly then I could reliably transmit from either first and it will work.
Anyone have any idea as to what is going on here?