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.

@reindier I discovered mysensors while also looking if it was possible to build my own alarm system.
I am pleased to report that it is quite possible if you don't mind the fun of building and fault finding.
I started a few months ago by building some PIR sensors and a serial gateway on a raspberry pi running Domoticz.
From there on I'm slowly adding to the system. I now have notifications for doorbell, outside temperature, solar panel voltage, sensor battery voltages, Sonoff light running Tasmota..... just this week I've swapped over to an ethernet gateway.

All the information is available if you take the time to browse the site. Lots of help on the forum as well.

@skywatch Appreciate the reply. I have made up some 500mA quick blow fuses that I already have from a trusted source.

@mfalkvidd I've just started to look into this sensor and the main page is still quite confusing as already stated by @skywatch

It seems the choice at present is between the BMP280 or BME280. the latter also includes humidity.
Maybe the title could say Temperature/Pressure/Humidity? I just ordered a replacement Si7021, not realising there was another all in one sensor available.
It is clear that most ebay sellers are also confused and are wrongly listing BMP devices as BME.
I found a good summary to the confusion here https://goughlui.com/2018/08/05/note-bosch-sensortec-bmp280-vs-bme280-sensor-confusion/
Hope this helps someone - Think I'll be going for a BME280 here as it as some positive reviews https://www.ebay.co.uk/itm/Breakout-Temperature-Humidity-Barometric-Pressure-BME280-Digital-Sensor-Module/401513062540

Breakout Temperature Humidity Barometric Pressure BME280 Digital Sensor Module

$3.54

266 available

@mfalkvidd Thanks for that clarification. I am also thinking of swapping from serial to ethernet when I have time so I can use MQTT.

Thanks to all your replies I think I've got it fixed.

I tried another pro mini....still eratic.
I then used that second pro mini and built an identical node on another breadboard. Result was perfect readings.
I then rewired the problematic one and gave it another battery for good measure. It is now more than acceptable although still not as smooth a graph as the second node.
At the moment, running direct from 18650 and not boosting to 5V does not seem to be an issue for the HC-SR501's.
Maybe I will run into problems as the battery voltage lowers.
Node2

Node1

I have now had my delivery of the above ebay sensor. Everything looks good. It is a BME280 and reports temp, humidity and pressure.

I have used the sketch on the build page and it has worked without any editing. I just had to install the BME280 library as given in the example sketch. Everything is shown nicely in Domoticz.

Interesting to note that when compiling in arduino IDE I get the following error : Low memory available, stability problems may occur.

Wonder if there will be enough room to add my battery level monitoring code?

I have my ethernet gateway up and running on a breadboard. It worked first time

If this is of use to anyone, all I did in Domoticz was go into Setup/Hardware and change the existing serial gateway to "Mysensors Gateway with LAN interface" and press update. All the sensors are then remembered as the Gateway IDX stays the same.

@mfalkvidd Sounds reasonable. I am thinking a small UPS would be a good idea so I could keep my raspberry pi's, router, switch etc going as well.
Time to look at that relay node then!

@mfalkvidd I've got it up and running now on the bench complete with measuring voltage. I found that removing all the serial print statements also free up a lot of memory. Thanks to all those involved for the provided code.

@mfalkvidd I've got it up and running now on the bench complete with measuring voltage. I found that removing all the serial print statements also free up a lot of memory. Thanks to all those involved for the provided code.

@mfalkvidd Thanks for that. This is going to be my outside weather node running off a 12V solar charged battery. I started wi a DHT22 but it failed after a few months.

I have now received 2 new Si7021 ebay boards. One 3.3V and one 5V. Both happily working straight away.
I have incorporated one into one of my motion sensors so I now get motion, battery level, room temp and humidity.
At first my pro mini would not sleep and just kept looping in the serial monitor - but as soon as I removed the FTDI board it started to behave? Somehow the PIR kept getting triggered. maybe too much going on with everything connected on the 3.3V line.

Anyway very happy with the finished sensor.

I have now had my delivery of the above ebay sensor. Everything looks good. It is a BME280 and reports temp, humidity and pressure.

I have used the sketch on the build page and it has worked without any editing. I just had to install the BME280 library as given in the example sketch. Everything is shown nicely in Domoticz.

Interesting to note that when compiling in arduino IDE I get the following error : Low memory available, stability problems may occur.

Wonder if there will be enough room to add my battery level monitoring code?

@mfalkvidd This is good to know. This may well be the time for me to learn a bit of coding with the fastled library.

@mfalkvidd
Thanks that was useful and I went ahead with the sketch shown here: https://www.domoticz.com/forum/viewtopic.php?f=42&t=8039&start=40#p

It works !!

Having searched the forum I can only find outdated information and links to old posts on the Domoticz forum.

i would like to be able to control my WS2812B LEDs using mysensors / domoticz / node red.
There are plenty of guides around for ESP8266 but I rally fancy adding this into one of my mysensors nodes.

Does anyone have an up to date, simple sketch to get me started? ie somehing that will simply let me change the colours of all LEDs at once and hopefully control from Domoticz.

Thanks

@mfalkvidd I've just started to look into this sensor and the main page is still quite confusing as already stated by @skywatch

It seems the choice at present is between the BMP280 or BME280. the latter also includes humidity.
Maybe the title could say Temperature/Pressure/Humidity? I just ordered a replacement Si7021, not realising there was another all in one sensor available.
It is clear that most ebay sellers are also confused and are wrongly listing BMP devices as BME.
I found a good summary to the confusion here https://goughlui.com/2018/08/05/note-bosch-sensortec-bmp280-vs-bme280-sensor-confusion/
Hope this helps someone - Think I'll be going for a BME280 here as it as some positive reviews https://www.ebay.co.uk/itm/Breakout-Temperature-Humidity-Barometric-Pressure-BME280-Digital-Sensor-Module/401513062540

Breakout Temperature Humidity Barometric Pressure BME280 Digital Sensor Module

$3.54

266 available

Having spent some more time fault finding, it seemed that the logic level converter chip had been soldered on wonky and one of the pins was not connected.
Ive unfortunately damaged the PCB in an attempt to fix so a new one has been ordered from another chinese seller.

Having now had time to read futher about the GY-21 is appears it will work with both 3.3 and 5V microcontrollers.

Interesting to see there is a GY-21P board that includes a pressure sensor. I would be interested to hear from anyone who has used this.

Edit 9/10/19 Just an update on this.
I now have new sensors on order Si7021 (both 3.3 and 5v boards) and also a BME280 which has temp, pressure and humidity.

@Yveaux I have just tested on a second board. This time a 5V arduino uno. Same result - as you say at "while (not sensor.begin())" call.
This board has SCL and SDA pins as well as A4 and A5. I have tried both.
Looks like a duff sensor then