Rover 1.4L 2000Reg

Hi I really need someones help here!I have a Rover 45. My problem is that that the coolant level goes down dramatically whenever I put the heating on. There is 55k miles on the clock and the water pump, belt and Head gasket where all done at 40k miles. If I don't turn my heating on everything is fine. However, my heating only blows cold air out and the coolant level drops when I put the heating on. I have no water in my oil or oil in my water, the car never overheats, the temp gauge needle never moves up just stays in the centre of the gauge. The only thing that happens is that the heater blows out cold air and my coolant level drops. I've been told it may a airlock but I've no idea how to get it out. Please help!