Help, How can I fix heating system running hot in ford puma 1999?

Despite putting car in garage twice to diagnose the problem and paying to have a new button on contol panel I've had no joy in sorting interior of car becoming really hot when driven. a/c doesn't come on, air vent blows out only hot air if I turn it on. Am determined not to take up £162 quote to fix this. I've researched new conrol valve on net think this may be problem, any one have any tips - besides drive with windows open which I have for 18 months. Ta Deb