Many thanks for the ideas.
The watchdog timer sounds interesting.
Re Did you update via the Setup > Administration > EmonPi/EmonBase update feature? I had but I can’t recall if I had done it since I applied the WiFi patch.
It mainly said up to date but did replace five files.
I have rebooted it again for a clean start.
Are these message normal for startup?
LAST ENTRIES ON THE LOG FILE
2018-04-11 19:53:11.026|ERROR|phpmqtt_input.php|exception 'Mosquitto\Exception' in /var/www/emoncms/scripts/phpmqtt_input.php:125
Stack trace:
#0 /var/www/emoncms/scripts/phpmqtt_input.php(125): Mosquitto\Client->connect('localhost', 1883, 5)
#1 {main}
2018-04-11 19:53:11.117|WARN|phpmqtt_input.php|Not connected, retrying connection
2018-04-11 19:53:17.009|WARN|phpmqtt_input.php|Not connected, retrying connection
2018-04-11 19:53:17.010|WARN|phpmqtt_input.php|Connecting to MQTT server: Connection Accepted.: code: 0
2018-04-11 19:53:25.565|WARN|phpmqtt_input.php|Not connected, retrying connection
2018-04-11 19:53:25.584|WARN|phpmqtt_input.php|Connecting to MQTT server: Connection Accepted.: code: 0
Time will tell if it behaves, if not I will try one of the fixes suggested.