Found this on an RV motorhome site...which explains why Butano does not perform well at low temperatures...
If you have a heavy draw on the regulator (total gas demand maximized) in cold temperatures (winter) as the vapor is drawn off, it causes the remaining liquid in the tank to become colder, and, as the temperature falls, butane becomes less and less effective. Butane remains a liquid, which means it will not give off any gas vapor at 31 degrees F at atmospheric pressure, while Propane continues to perform at a higher level at the same temperature - it liquifies totally (stops gassifying) at -48 degrees F at atmospheric pressure.
Heating a gas boiler obviously puts a high drain rate on the tank and so it gets even colder than the ambient air around it.
Propane is the gas of choice under these conditions but whether its freely available is a differnt matter of course.
Many moons ago I eas lodging with a guy in a caravan in Reading and his Butane tank used to freeze up almoost every nght, since he used the gas for his living room heating it was a real problem keeping warm.