Right, at long last I fixed this. I’ll record my stupidity here in lieu of a public flogging.
I had a number of ESP32 devices - some test ESP32 setups on breadboard and others were loose modules. I had previously setup my two Dallas probes on the breadboard ESP32 and imported them into HA and they worked fine.
I then moved the same two probes over to a loose module in preparation for deployment at my central-heating system. The problem that I didn’t realise was that the individual Dallas ID numbers were still registered with the breadboard ESP32 and when the new ESP32 tried to register them in HA it failed due to the duplicate ID’s. I had assumed that the ID’s were not passed to HA and that only the names were, but how wrong I was.
2023-01-21 11:09:51.710 ERROR (MainThread) [homeassistant.components.sensor] Platform esphome does not generate unique IDs. ID dallas-003c01e076ed8c28 already exists - ignoring sensor.xxyy_temperature
2023-01-21 11:09:51.713 ERROR (MainThread) [homeassistant.components.sensor] Platform esphome does not generate unique IDs. ID dallas-8b3c01e076cbaf28 already exists - ignoring sensor.testsensor02
I had blissfully ignored the errors above as “they aren’t my new sensor names”!
I finally copped it and deleted the breadboard device from HA and re-imported the new ESP32 with the sensors attached - et voila!
t1 and t2 are sitting there thumbing their noses at me.
Lesson Learned: Dallas probe ID’s are individually registered with HA. Read the logs, again.