The only really reliable way is to heat your flat when you're not in, I think. It doesn't need to be as warm as you'd have it yourself if you were in, at least.
I've had most success by leaving windows open if the weather is cold and dry but it's obviously not a permanent strategy in the winter because it's bloody freezing. You can get closeable trickle vents for your windows which might help if you don't already have them (or you can hire somebody to put them in for you, obv).
Edit: oh, i spent too long eating cake and not enough time typing :(
The only really reliable way is to heat your flat when you're not in, I think. It doesn't need to be as warm as you'd have it yourself if you were in, at least.
I've had most success by leaving windows open if the weather is cold and dry but it's obviously not a permanent strategy in the winter because it's bloody freezing. You can get closeable trickle vents for your windows which might help if you don't already have them (or you can hire somebody to put them in for you, obv).
Edit: oh, i spent too long eating cake and not enough time typing :(