Best thing you can do is to take a weekly gas reading to check your usage for 2 weeks or even do 2 monthly usage checks. Run your system as normal for one period and then try using it with the constantly on, low heat setting.
It also depends what boiler you have, We had one house with a 20 year old wall mounted Potterton boiler with the seperate tanks and Emersion Heater - This system was cheaper to have running constantly.
With a Combi boiler system we found it cheaper to just use the timer switch for an hour in the morning and 4 or 5 hours in the evening.
I have the opposite problem now - trying to run the air conditioning economically for about 9 months of the year, can't wait for roof mounted solar power systems to get cheaper !