Only the more expensive weatherproof covers give much in the way of protection. To a greater or lesser extent they produce condensation within the micro-climate they create. And the cabin itself also creates its own micro-climate which will lead to dampness and, eventually, mould.
The key to preventing this is a continual movement of air and, to this end, the exterior weatherproof cover does nothing to prevent that and - indeed - can cause damage to the car's bodywork if wind moves its interior surface across the paint.
Cars are designed to live outdoors and the A40 is no exception (*but see below). The simplest way to ensure air movement is to leave both doors' windows cracked open. The Mk1 glass locks when lowered about an inch providing the interior door handle is in the locked position; the Mk2 glass, with its winding handles, can just be cranked open to allow that inch gap which is all that's necessary.
To prevent driving rain penetrating the air gap, obtain a sheet of sturdy baking foil. Cut a piece 34" long by nine inches deep. Fold that dimension three times to make a 3 inch deep strip.
Position the foil so that the top stands above the door's top edge and fold it inwards and down so that it meets the rubber sealing strip fitted to the door.
Next, move the entire strip so there's about an inch overhanging the rear of the door and fold that inwards to meet the same rubber sealing strip.
Now carefully fold the overhanging foil along the angle formed by the slope of the door's front edge. You will need some scissors to trim the excess in order to then fold the foil, as before, over the door frame to meet the rubber sealing strip.
Repeat for the other door.
You now have weatherproofing items which allow air to enter the cabin while still preventing rain from doing the same. Naturally they would need to be removed before driving the A40 but, with care, the strips should last a while (and being so simple, can easily be replicated).
I used this trick many years ago when Gab had to live outdoors when Bar & I married and we were broke. Some years later we bought a house with a garage. Of course, during wet weather the air itself is wet but not carrying physical droplets of water; nonetheless the cabin should air itself when the humidity decreases.
*The biggest danger to an A40 parked outdoors is a build-up of detritus like leaves, moss etc falling through the grilles at the lower edge of the front windscreen. The plenum chamber into which they drop has two drains specifically formed to allow water to leave it but, of course, those drains must remain clear if they are to work efficiently.
They emerge into the engine bay as two very shallow indentations pressed into the bulkhead: one can be found to the right of the heater unit between the wiper motor, while the second one appears on the other side of the heater by the master cylinders.
A wire coat hanger, straightened and suitably curved, is a good probe to unblock anything lodged inside the chamber and, for an outdoor car, it's sensible to check the drains free-flowing a couple of times a year and certainly before the winter closes in.
Blocked drains will allow trapped water to rust out the bottom of the chamber and this is a tiresome and expensive job to repair, usually necessitating the removal of the dashboard and the wiring behind it. Some A40s may already be suffering from this (a damp parcel shelf is the usual giveaway) so if one wishes to flush the chamber through with water, it's sensible to lay a towel along the parcel shelf before commencing operations.
