Curious. I'm getting the following so it looks like some sort of networking issue:
2020-08-22 09:19:38: error fetchTextFromWeb:Net Exception: No route to host
2020-08-22 09:19:38: Execution of webAction e3pon failed. Error Code:400 msg:
I can't ping the address of the sonoff which is very odd as they are both connected via the same access point (and I can from a laptop connected to the same WAP).
At least I now know where to look so I will run some further network diagnostics.
Thanks for your help.