I'd like to close this topic now with a thankyou to all who helped and an attempt at a summary.
PiHole is working again without generating that 'Ignoring query from non-local network' message. The modification pihole checkout ftl new/non_local_details was helpful in showing the IP address that generated the message. It was a wifi device on the same network as everything else!
I have, on my Raspberry Pi, both the Ethernet and Wifi connected to the same network. On the DNS tab of the Settings page I have changed from the default to Respond only on interface eth0. I temporarily disabled the WiFi (with sudo ifconfig wlan0 down) but that didn't seem to make any difference.
I am now awaiting the fix to PiHole -chronometer. If I was more impatient I would apply the fix, but I'll wait for the official fix to come through as part of the build (probably using the wrong terms as I know nothing about Github).
Thanks again