how do I sort out a digistat+3 thermostat that is malfunctioning?

I moved into a new build property and have a Drayton Digistat+3 thermostat and for the past year it has failed dismally to maintain a regular temperature despite being set on manual at 19.5 degrees/24 hour period.It regularly goes on when it already is displaying a temperature above the set temperature or it will not come on when the temperature is very low.Opening a door or window however briefly sends it into overdrive as any change in air current will activate it.Walking past it will show a change in the temperature.So I'm struggling to live with temperature variations between 16.5 degrees and 24.5 degrees.Due to ill health I need to live in a constant temperaure not swinging between freezing and a sauna.There are TRV'S on the radiators and the boiler is a combi boiler .....Ideal Isar.Despite my complaints the company who installed the thermostat they totally ignore my complaints.Last week they took the battery out and just put it back again apparrently to re set it.There has been no change in it's malfunction.It has to be faulty so where do I go from here.I would pay to replace it even though I am in a rented property and that is actually the landlord's responsibility.