I guess that the Hive receiver uses a different protocol than home-automation. I had been hoping to just purchase the receiver of ebay. Wiring it into the thermostat contacts and then use ST Therm app (or write something similar) to avoid the cost of the Hive or Nest Thermostat.
As I understand it the Receiver is only a volt free Zigbee switch (with a certain current rating). Replacing the simple volt free contact of a transitional Thermostat. Nothing special.
In theory I could fit any Zigbee volt-free switch (or Zwave I suppose). But I wonder if that would contravene any Gas regulations.
I recognise that for most people it is better to just purchase a Hive/Nest and be done with it. More reliable.For me I would enjoy playing with scripts to replicate what the Hive/Nest do.
But it does not seem that the Hive receiver is the low-cost way to do this as I had hoped. So thank you for your help.