It appears that there's a bug - at least as far as the MG5 is concerned.
Starting the AC using the ON/OFF button in the new app, the system ignores whatever temperature is set and just pushes out air on LO - so useless for preheating the car. Tried the old app, and it all works perfectly.
There seems to be a work round in that if you don't turn the AC on and use the right hand arrow to go to the HVAC screen, you can set the temperature and press SEND. When it's worked, it also turns on the AC at the temperature that you've just set.
So - what is the best place to report this bug?