Air Con and Heating on all time in 110

I've a factory fitted air con unit fitted to my 2000 defender 110. Presume it was fitted when 110 was new (I bought it 2nd hand).

Problem is that the heating is on all the time - the lever to right of steeting wheel that on my previous defender (without air con) is fixed down to full heat all the time, I can't move it up, presumably bolted down.

Great for winter driving, a bit hot when it is 30 deg as the windscreen vents etc. are churning out hot air all the time.

Is there any way I can get control back over heating?? Air con doesn't currently work (needs servicing and not planning to do that just now, managed without it for years in old Defender, no plans to get it to work now), so I don't care about overriding it.

Undo the screws that attach the heating controls and take a look inside, only takes a couple of minutes, you might find the cable is seized or the cable is stretched. Then check the heater box side to see for free movement of the flap spindle, that might be seized also.