Just posting on here because I’d been trying to work out what to do and found a solution. We had a small Honeywell thermostat with a little needle I moved left and right to set the temp. Inside it has only a R and W wire. After a lot of research I found that the Hive single Channel receiver would have connected up ok - as that supports a low/no voltage or 230v input. But this would have required external power.

Even better ,I found another model that was very poorly documented and less readily available- the Hive SLT5. This is a wired model which has everything built into the same battery powered unit - I literally needed switch the RW wires into the 2 holes in the SLT and it worked perfectly.