I have experienced similar symptoms, suspected head gasket failure but found the cause to be far more mundane.
Thanks to Laird Scooby for clarifying that these experimental mixtures the greenwash brigade would have us believe are petrol contribute to excessive winter exhaust vapour. I always use super-unleaded which is supposed to be capped at 5% ethanol. I expect it still contributes to this effect though.
In my case there were multiple culprits for slow coolant loss and a soggy engine bay.
- Suspect expansion cap
- Leaking small hose between the top of the radiator and the expansion tank
- Cracked radiator side plastic tank - only leaked when hot
- Newish oil cooler hoses needed the jubilee clips tightening
I fixed all these myself relatively cheaply. A new Nissens radiator was only about £100 from Eurocarparts using the right coupon code.