if you like I can give you the service number you could call. I might be having the same difficulty. Mine wasnt working for ages, thought it had packed it in but turned out that it was just the thermostat control that had tripped. Easily solveable, TG.It is giving out heat but considering the temp outside and that water is not flowing in pipes due to freezing it can be that it should be turned up to the max. i.e Input level 6.
Here is the number, email ( not allowed any emails b the site sorry )
0870 7270 101
Paul, January 2010