The heater unit. This is a 77 and later version with the two-speed resistor fitted inside the heater unit. The earlier single-speed is much the same but without the resistor wiring and just two wires connecting to the main harness with bullets instead of a 3-pin plug and socket. (Image from Paul Depper)

Back of the heater unit, pre-77 with just two bullets connecting the motor to the main harness. This is a very early unit, later units had a mesh screen over the cold air inlet to prevent leaves entering from the air-box and clogging the heater unit. The circular foam seal prevents fumes from the engine compartment being drawn into the passenger compartment. (Image from Leyland Parts Catalogue)

Clausager has this very useful photo of the Gaydon cut-away engine on page 62 which shows the heater tucked away in the corner of the picture. I've labelled it with its main components and the body structure around it, plus the separately operated cold-air flap. Unfortunately the air direction flap appears to be missing from the heater unit.

The next three pictures show how the air flap controls direction. In this position both demister and footwell vents are closed off.

In the central position of the dash control the flap is straight up and down. On the face of it this appears to direct air to both demister and footwell. However the longer, narrower and more tortuous past to the demister means relatively little air escapes this way (it can be felt), the majority does flow to the footwell. The demister deflectors are only positioned immediately in front of the outlets, so either side of those air is going to go straight down anyway.

Flap turned to the other extremity closing off the footwell vent, all air being directed to the demisters. This begs the question of why the demister deflectors are there at all, as the flap in this position makes them redundant. You could say that they found some air to the demisters all the time was beneficial in keeping them clear, and the deflectors were needed to make sure some got through. But that is surely asking a lot for BL/Smiths in the 60s!

The passenger side in the V8. The flap is shown in the Interior position i.e. more or less straight up and down. You can also see some foam strips which are beginning to come adrift.

The driver side and the cable for the air flap. It can only be shown in two separate pictures on an RHD as the pedals get in the way. Note the rubber block, the clamp for outer is covered by it, making undoing or fastening without cutting into the block impossible.

The inner fastening to the flap lever, shown in the Off position. This looks to be more accessible but I can only just get a finger-tip to it, although access would be better with the cowl removed. You would still need to manipulate the end of the cable with a pair of very long nosed pliers, and the trunnion with a nut driver on an extension, you wouldn't be able to see the hole in the trunnion, and the pedals on an RHD prevent you looking straight into the space! All in all a difficult proposition even if you were only replacing the inner. This seems to be a stranded inner, but logic dicates that it should be solid as it has to push the flap closed as well as pull it open, and there is no spring to aid closing. I must be fortunate in that this flap is very easy to move. Interestingly my mate Terry with a 73/74 reports that his also seems to be stranded as is my 73 roadster, whereas the replacement from the MGOC (original chopped off both ends when the heater was removed by the PO) is solid.