Which Room Thermostat do we need?

We have just had gas CH put to the property but there is no room thermostat with it!
The Boiler is an Ferolli Optimax HE Plus 31 C and we need to know which thermostat will do the the job.
Can anyone advise please because without this,the boiler is working contantly because of the temperature outside:ie:(snow)
Thanks in advance for any help.