Whilst thinking about all these heater issues us 75 owners seem to get faced with, another light bulb in the head went ping
Right, here goes...
This top tip should, hopefully one day save you a failed mot for 20 mins work (providing your car hasn't allready done the dirty)
Open your bonnet, lift the rubber seal at the back (which seals your bonnet from the panel your wiper motor hangs from) unclip the sound deadening screen from this panel (which hangs under this rubber)
You should now be able to see your bulkhead, pedal box, wiper motor etc)
Now hopefully the centre region of the bulkhead has not rotted out (common on 75's) if it has, its an MOT failure!
Right, heres the good news if it hasn't, this is best done with an old hoover or something similar....
where the heater opening is there will probably be an acumulation of dead leaves etc... get rid of them! they harbour moisture which causes the bulkhead to rot out! And make sure the drain pipe is not blocked (on bulkhead towards o/s in the engine bay)
Replacement panels are no longer available, and not easy to replace even if you do get one (engine out, heater box, dash OUT!)
Area concerned is the top middle of picture (black painted bit)