Heater problem

On my '91 disco V8,I can get warm/hot air when the levers are all set to hot and to the windscreen,but when I move them to the front or footwell vents there is only cold air blowing.It's great for clearing the screen at 6am to go to work,but no good when I want to warm myself up too!Any ideas warmingly welcomed as the temps start to drop outside.:flame:

Am sure there's a flap near the heater matrix, see if you can see where the cable goes?