When you say the windscreen demist has been less efficient, do you mean with the aircon working, or without? Or is the heater just putting out less heat than before?
In all honesty I've never knowingly had a head gasket go. Years ago a family member did with an original 3 Series BMW and I think one of the symptoms was it running hot, though also we thought the problems began with a faulty expansion tank cap that wasn't holding the pressure, that also caused overheating, which in turn caused the head gasket to fail. So unless coolant wasn't flowing through your heater pipes for some reason, I'd have thought even with a faulty head gasket you'd still get heat into the car. In the old days, with an engine oil dipstick, you'd get emulsified oil (mayonnaise) on the dipstick if water was mixing with oil. Maybe you can see this under the filler cap if there's no dipstick. Also, you might see air bubbles in the coolant in the expansion tank with the engine idling if the cooling system is being pressurised.
Unfortunately there's never any guarantee other issues might not be found when a car is taken apart. That's why I asked if the demister issue concerns air conditioning not working, or the heat going off.
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