fiesta 2006 overheating

 Hi guys, just joined to see if anyone can help me here. My 2006 fiesta is overheating (ie:the temp gauge rises above the half way mark) after about half an hours driving at 3000rpm. The temperature gauge comes back down if i turn the heating in the car up to at least a third into the hot (red) section of the heater temp control dial.

 The problem started about two months ago when the warning light notified me that the car was overheating, and it was indeed, the dial was into the max. turned heating on to bring temp down and got it into garage asap. The water pump had seized up and was replaced. Getting the car back, it was bit better, but still overheated after driving for 45  mins or so, and usually when going up a big hill or hitting 3000rpm for a couple of mins.

 So i drained the water in the radiator incase of airlock. Didnt fix problem. 

 Car back into garage where they bled the system (i think reverse bled) including the radiator at front and apparently a smaller radiator at back of engine block which is what heats the air in car apparently. This didnt fix problem either.

 I have noticed now that the days are getting warmer, the car is overheating quicker, so I am driving with the heater on a lot, with the windows open to help cool me down. Obviously this is not ideal.

Car is booked in next week to get thermostat housing changed.

There is no water leaking anywhere, and the oil is not cloudy, so i am hoping its not a head gasket issue. The garage doesnt believe it is.

I have also noticed that the reading for the outside air temperature has gone up since this problem started. ie: when it is 15 degrees C outside, its saying about 22.

 Can anybody help me please. Im not happy doing long journeys at the moment, which is kinda going to ruin my holiday in two weeks! argh!

Thank you very much.