The Delanair Mk III is microprocessor controlled and is a good system while all is in order, however if you lose your refrigerant for whatever reason there is a problem in warm weather. Due to the car trying to cool itself, it will close the fresh air inlet flaps and open the re-circulation flaps. This means that the car will get more stuffy not cooler, a vicious circle.
There is a simple work-around to get fresh air again, disable the recirc system. Of course you will still not have refrigerant cooling or de-humidification, but it is an improvement. Remove the left hand cheek in the footwell.

The metal duct with the 'do not lift' sticker is then removed by taking out the two small self tapper screws (one can be seen in the photo), this reveals a confusing amount of wiring, but despair not!

Follow the blue plastic vacuum tube and it will lead you to the re-circ solenoid. Pull off the connector which has a red/blue wire to it. Tape it up so that it won't get lost or short onto anything and you are done! Literally a 15 minute job. As I said previously, don't expect perfection, but it will make driving more comfortable until you get that pesky refrigerant repaired.
