OK, I think I’m done!
The last part of the jigsaw was to turn the switch off after an hour - thus creating a “Boost” button rather than a “Turn the heating on, let me forget about it, waste all my money and turn my house into the centre of a volcano” button.
I went with Rules Machine - Rule machine - as per the app developer, this app is no longer available for new installs, distribution, or support
It was easy to install (Heck, I did it and it worked first time, and sometimes it takes me a few goes to get my shoes the on the correct feet in the morning) and provides a similar sort of approach to IFTTT, but with multiple conditions/triggers and more actions.
I just went ahead and made a new rule that basically said “If the Boost Switch is on, then Turn the Boost Switch off (with a delay of 60 minutes)”. The delay is something native to Rules Machine, so that was nice and simple.
End result is I hit the button, I get a text message from Hive confirming my heating is set to 22 degrees, and then 60 minutes later I get a text saying it’s gone back to whatever my schedule says it should be.
Done.