Clive,

When I've had the heater control valve fail it's resulted in A:coolant leaking on to the driver's side floor boards or B: heat even when the valve was in the full off position (no fun in the desert summers). In your case it could be the control cable on your valve is not connected or out of adjustment. Pull the under dash kick panels on the driver's side and check the action of the cable on the valve as you work the dash slide control. The valve is located in the upper right hand corner of the foot well ... just to the left of the heater/blower enclosure. Copper valve with two rubber coolant hoses and the control cable connected to it. Make sure when the dash control slider is in the full heat position the valve is being pulled full open by the wire. You can lenghten or shorten the cable in it's clamp to adjust. Could be when the A/C work was done the cable may have been loosened or disconnected. of course your problem may lie elsewhere but check the valve action adn cable first.That's the simple fix.Hope it's what your need.