The fresh-air vents in the 73 and later UK dash. To remove them (one at a time, they are individual) carefully lift up the tabs (arrowed) at the inner corners with something like the point of a knife blade - one at a time whilst easing the bottom (if you are lifting the lower tab) of the vent forwards, then lift and ease forwards the other tab and upper edge:
It's probably easiest on RHD cars to remove the left-hand vent first as you can reach up behind the dash on the passenger side to grasp the body of the vent to do the easing forward bit while you are lifting the tabs. Malcolm Whitehouse sent me this info on a tool he made to make it easier - and ally rod with the tip flattened into a chisel-shape:
Shown inserted with the vent removed for clarity:
The WSM says to remove the tube with the vent, but the tube seems to be clipped into the bulkhead whereas the vent is just slotted into the tube, so it's much easier to leave the tube behind. Once that one is out you can reach the right-hand vent through the resultant gap.
To replace, do the right-hand one first as you will need to position the rectangular end of the heater outlet tube inside the open end of the dash vent, and this is easiest done with the other one out of the way. To refit the left-hand vent reach up behind the dash as before, but this time to position the heater output tube in the vent: In case you are wondering about the three 'foreign' switches, a PO had rather butchered the dash to fit a couple of switches that were neither the same as the originals or each other. I was fortunate to get three illuminated switches of the same type but different colours/logos to tidy it up a bit. The left-hand one with the fan logo is connected to the cooling fan relay, so not only can I override the thermostatic switch if I want to but it also shows when the thermostatic switch has closed to operate the relay and fans. The middle switch has red illumination and is used to operate the rear fog lights. The right-hand switch has green illumination and is used to operate the front fog lights.
November 2016: The section from the manual, with the parts about the packing boxed in red by Ray:
One of Vee's vents with thin strips of card glued to the back of the flanges - arrowed. There is a third one on the other long side:
The factory packing would have to be thick enough cater for both flanges (A), but if wide enough and too thick it will press on the pivot and limit stop (B) and could prevent the adjustment that directs air up or down:
The tubes BHH609 that connect the air vents to the bulkhead. NLA from the usual suspects at the time of writing, some showing up on eBay. The round end is a push-fit into the bulkhead hole, each with four protrusions (arrowed) to click them into position. The vents themselves are just a sliding fit onto the square end:
The cowled inlets in the heater intake:
