@Wayne-TP 

I discovered that shortly after I wrote the query.  I thought "lets be silly and write 2 independent control rules and let's set them both trigger at 15 deg (that will confuse the system)", I was getting annoyed.   Son of a gun, it worked and was not confused by having the same temperature.  I also discovered that if I want to turn the heater off during the day I had to write a third control rule for the P110 plug.  So it's all working now but I do think the way I had to implement it was neither obvious nor elegant. It should be a single line for a very simple control command where a state changes either side of a single setpoint, this will be applicable to many things not just heaters.  For example, I don't think Tapo have a light sensor but it would be good to be able to program lights to come on when it gets to a certain level of darkness (dusk) and go off at a certain brightness (dawn).

I feel I should say that apart from the above poor experience, I have been impressed that the cameras, sensors, lights and switches I have bought recently have mostly installed easily and quickly and they mostly integrate well with Amazon Alexa. There was a major bug early on but that was because was auto-filling my WiFi password and it had a trailing space I didn't notice.  When I realised that, the installations have all gone well.  It may also help that I have a very stable, fast TP-Link mesh WiFi.