Some boiler installations have a "frost stat" to turn the system on during very cold weather to stop the pipework freezing up.
I do not know if this is the cause of you boiler coming on - a qualified engineer may be able to disable the frost stat for you - my guess is that they are all very busy just now and they all charge for their services.
During the day I keep our hall mounted room stat at about 17 degrees and at night time when retiring tweak it down to about 12 degrees - I do not have the luxury / annoyance of a frost stat.
I am not qualified or experienced to offer sensible advice.
Peccavi, December 2010
