Hi Vicky. It is quite common for customers to experience heat loss come the winter. Often we attend situations like this and it can be the fact the garage underneath hasn't been insulated, it can be missing cavity wall insulation or just poorly laid loft insulation. Draughty windows tends to be an obvious one customers can spot due to the wind. If you haven't already addressed the situation, I'd suggest going into your loft and safely see if there are gaps. Make sure you are careful to only walk on the joists when up there. Sometimes gaps in insulation are caused when other trades go to do their work on ducts or spotlights and throw the insulation aside. It does not need to be much of a gap for heat loss to be significant, and most of your heat will escape up through the loft. You also want to check the insulation is cross laid, meaning you cant actually see the joists.
Failing this, you realistically need specialist checks to the cavity wall or garage roof. You can opt for a professional thermal imaging company if we are booked up.
Best wishes in the meantime