Short trips, lights on, wipers on, heater on
Very cold weather
Engine taking longer to get up to normal running temp
= less MPG
I'm not sure it's all down to the temperature, unless you're doing lots of short journeys? Increased alternator load wouldn't account for a nearly 30% drop in economy and once the engine is up to temp, it should give similar MPG despite the outside temperature.
FBH does make a difference, but only a little one - doesn't it use less than a litre an hour?