Hi Noiasca, thank you so much, your help has been invaluable! I have changed both the Master IP and the Gateway IP to be the same (I used ...4.1) and now all of my boards work as either a Master or as a Slave.
(I am still slightly confused as to why one board worked previously and the others didn't when the code was obviously wrong. I would have expected that all boards would not work. This seems very strange indeed).
I have now been able to move the project on some more as a result, so thank you again, but I now see one more anomaly. I have both Slave 1 and Slave 2 now running and Slave 2 works fine on it's own but when I power up Slave 1 it's messages do not get through to the Master. It seems that whichever Slave connects to the Master first takes over and the other Slave (although it connects to the Wi-Fi) does not get heard by the Master.
I see that the default number of connections for a Soft AP (with an ESP 8266) is 4 so this should not be the problem (besides the Terminal Monitor response is reporting that both Slaves are connected to WiFi OK). Do you think there is an obvious reason for this problem? If not I'm happy to go away and do some more reading / research.
Thanks again for your help.