It's a bit of an issue. You can of course do this programatically if you have temperature sensors in all the rooms. But you'd have to drive the Horstmann relay programatically, which isn't ideal. The thermostat has some clever logic to drive your boiler in an optimal way (I think you can even set the boiler type in the Horstmann).
What I did: I put thermostats in the rooms that I want to have good temperature control in (3 rooms). The relay units near the boiler are simply wired in parallel: this defeats the cleverness of the boiler control... but only when more than one thermostat demands heat while being near its setpoint. The TRVs in those rooms are programmatically slaved to the thermostat: every so often the home controller reads the setpoint and updates the TRVs.
The TRVs in the other rooms are usually set to 14 degrees. At the moment I have them set so they go to 20 degrees when I turn the lights on in that room, and I close one of the boiler relays for 5 minutes to draw heat into the radiator. These are small rooms and this works well enough.
By the way, I'm getting some serious savings by not heating the rooms that are not in use during the day.