Right. So, latest state of play is:
Binned the simulated button, went for a switch.
Used IFTTT (since it works with switches, not buttons) to send a HEAT AUTO 22 message from my phone to Hive when turned on, and HEAT AUTO message from my phone when turned off. So I’m using IFTTT to generate the texts from my own phone, hence it’s a UK number, and they’re quite happy with that.
So the remaining issue is to work out how to toggle a switch back off when it’s been on for 60 minutes. I saw someone mention something about an energy SmartApp so I’ll go hunt that out.
Thanks all so far for help with this!