Hi Chris
To be honest we haven't found it to be too bad but have got a habit of cranking up the heating if it feels a bit nippy. Have considered getting a free standing heater just for the downstairs but only because I read somewhere it would be a lot cheaper than running the heating thru the aircon unit. Don't know if thats right or not.
The only other thing we did was to fit draught excluders on the front & back doors (mainly to stop the dust coming in over the summer - works to some extent) but also to obviously stop any draughts in the colder months. Find the floors flippin freezing - rugs & slippers a must!