Hmm, that was interesting.

I had cut the board to separate the two modules but in doing so I had isolated one ground track. The LCD ground was on it.
Turns out it was using the grounded switch, when selected, to power LCD so when selected switch to 1 it had no ground except via some sort of torturous route, hence resets. Now sorted.

I have gone back to DEBUG using the hardware pins and turned off the UART. It wouldn't receive from the rf module but would transmit. Puzzling ? DEBUGIN works so back to a working system except data for RSSI.

I still get a compile error for the DEC3 even though I tried both syntax versions.

Rob