Vee's heater (removed as part of a restoration) cover removed, manky foam on top of the matrix ...
... and no foam at all on the other side, allowing a significant amount of cold air to bypass the matrix.
New seal fitted, seals the ends as well as the sides, so air can only pass through the heater unit by flowing through the matrix itself.
However I note that the late Leyland Workshop Manuals show the foam seal wrapped round the upper half of the matrix, with the ports poking through:
Maybe saved them from fitting the port grommets below!
Seal for the matrix ports where they pass through the cover. A grommet, so fitted to the cover before it is fitted over the ports.
By cutting across the cover immediately below the bigger flange that screws the heater down onto the bulkhead shelf (red arrows), the cover can be removed with the heater unit in-situ. One corner of the matrix is still tucked down into the bulkhead shelf slot, and there are parts of the main case that look as if the matrix can only be withdrawn straight forwards. Presumably it can be fiddled out otherwise people wouldn't do the mod, although I don't think I've ever read of someone subsequently needing to remove the matrix to prove it! The cut-off piece should be pop-riveted to the main case, and ideally a small strip across the bottom of the main cover that will go down into the bulkhead shelf slot in lieu of that part of the original cover, to maintain the sealing so that all the heated air passes into the cabin and doesn't leak out into the engine bay. If you are desperate and need to change the matrix without removing the heater it's said that you can fold the cover down, presumably far enough to clear the matrix tubes, but that would seem to allow even less space for the matrix to be wiggled out past the location pieces in the main case. The other thing cutting the cover allows is to fit an uprated impeller to the motor, which is said to be bigger than the hole in the cover unlike the standard impeller, as well as to be able remove the motor with the larger impeller.