Not a system I am familiar with but a quick Google shows the following ...
The Hive Thermostat Model: SLT3c seems to comprise of a receiver (which replaces the heating controller) and a thermostat (with the dial on the front and which you mount on the wall in a room somewhere).
It looks as if the thermostat and the receiver can talk to each other using their own communication network (Zigbee) which allows the thermostat to remotely control the heating.
Hive support page here which lists the model second lot of photos down
https://www.hivehome.com/guides/hive-heating
If you want to control the system via app/phone/iPad etc. you need the Hive hub by the look of it (first item here)
https://www.hivehome.com/guides/hive-connectivity
This is the bit that plugs into your router. It allows the wi-fi connection from your app/phone/iPad to link to the Hive system (through the router) and, in turn, network with the Hive's communication system. The Hive hub allows you to control your heating when you are inside and outside your home via the app.
So, if you were able to control your heating via iPad in the past with BT, then presumably you must have had the Hive hub to do this.
The Hive receiver (which replaces the programmer) will presumably be near your boiler where the old programmer used to live.
The missing Hive hub would also presumably be somewhere in or around the location of the old BT router.
Is it feasible that the VM tech packed it up somewhere with the old BT router when the new VM hub was installed?
If you had the Hive system installed by a plumber/heating engineer, why not refer back to them on what they installed originally and see if they will drop by to identify what is needed (and what is missing?!)
Any other Hive-owners on the forum may be able to expand on and/or correct my Google searching above.
