For future reference, under RRF3 the M305 command needs to be replaced with M308.

This is from my machine where I have a thermister connected to the second extruder (E1) thermister connector. The A term is the name of the sensor that appears in the extras list. The S value should be between 0 and 31 for a Duet 2.

M308 S10 A"Extra Thermister" P"e1_temp" Y"thermistor" T100000 B3950 R4700

More details at https://duet3d.dozuki.com/Wiki/RepRapFirmware_3_overview#Section_M308_Create_or_modify_sensor_or_report_sensor_parameters

This could explain why my delta's nozzle started engraving the build platform. It didn't happen every time, so I assumed it was my 4 year old, heavily used, printer wearing out, even though I couldn't find much slack. In the end I stripped out all the electronics to go in a second CoreXY machine. I didn't think to roll back to an earlier firmware version. Don't I feel silly now. I've still got the delta chassis, so I may refit it.

That would explain a problem I was having. My WiFi key is 41 characters long. I set up a second access point with a shorter ssid and key. Time to upgrade to 1.20 I think.