Update: I WAS missing something obvious! Sorry to anyone else I baffled.
I was using the 'other' Tx/Rx pins on the A6 module. Now that I am using the U_TX/U_RX pins, which are for the UART, it is working.
I don't know what the other pins are for, I haven't seen any mention of them.
I can see that the 'wrong' Tx pin seems to be sending at about 10x the standard 115200 baud.
I also found what has been mentioned elsewhere - that the software serial is unreliable at 115200 baud on a Uno. I have been able to switch to a lower speed and then it is reliable. I plan to use a Mega anyway.
I am also having some difficulty getting the A6 to initialise reliably, but have not yet had time to try the various tricks available.
Regards
Peter A