So after some considerable time and loss of sanity I now appear to have a stable setup. I'd like to share what I have done in the hope it may save others a lot of time and frustration.
By enabling debug in the sketches, I eventually realised that I could not get any node to communicate reliably with the gateway. I was getting lots of NACKS even at short distances.
I was sure that the problems were radio related but after trying many NRF24 modules I was about to give up.
Then I read a post somewhere about ground planes.
As a last resort I added a piece of aluminium between my gateway antenna and the radio. (PA/LNA Version). Eureka!! This has pretty much solved the NACK issues.
Following this one or two nodes still intermittently lost contact after a few hours or days. What is strange is that even if moved close to the gateway, errors continue until the gateway is power cycled? I still don't know why this is.
I solved this problem by converting the Node PCB antennas into a dipole antenna as detailed at https://www.instructables.com/id/Enhanced-NRF24L01/.
Now all nodes stay connected.
Now after a couple more weeks of testing all is still good.
I am using Domoticz with the MQTT output passed onto Node Red and now have some really cool things going on.
A big thanks to all that replied and pointed me in the right direction.