Many thanks to you all for the help:
I have tried various distances as you suggested and carefully followed the excellent instructions prepared by JJ604 and Daedalus66. Binding was successful at the second attempt using the Lemon 6 ch rx, the one with no hard casing which runs in DSMX - and the Spektrum DX6 G2 transmitter, no problem.
Nearly all my binding efforts with the 7ch rx have been holding the bind button down and switching on the tx with the rx powered up and flashing. After a short while the flashing on the rx stops altogether and you get the doleful voice: 'Bind failed'. I did try a couple of times using the tx menu bind function but that didn't make any difference.
The casing of the rx has text in bold yellow: 'Lemon RX', followed by smaller white text 'DSMX 2.4Ghz'
The fact that the 7ch rx binds normally with my other tx, the Spektrum DX6i but in DSM2 (the tx only works in that) means I suppose the darn unit(!) is not faulty. But there's no telemetry facility on a DX6i
Do you think then that Gangster's point then may be the solution: the rx doesn't actually work in DSMX but does work in DSM2?
If there were someone among us who actually had the rx working with a DX6 tx, I'd be much happier!