OK don't know why I didn't think of it before, I used my scope on the TX RX pins when both units are in test mode.

They have data back and forth at 1 second intervals. That means they are transmitting and receiving.

So, it's down to this routine not working. Given the details above I just can't see why :-

Code:
sigstrength:

    IF jump = 1 THEN rssioff     ; check input to reset to serial                                             
    DEBUGIN  3000,sigstrength,[WAIT("aSSRSSIS-",sigfield)]    ; get slave signal strength value                                        
    DEBUGIN  3000,sigstrength,[WAIT("aMMRSSIM-",sigcontrol)]  ; get control signal strength value 
    
    LCDOUT  $FE,$80," Light Unit ",sigfield                                       
    LCDOUT  $FE,$C0," Control Unit ",sigcontrol                                         
    GOTO sigstrength