I found 2 more sources of potential air leaks for the heater fan air supply under the
dash.
Firstly the plenum chamber supplying air to the
dash vents has been sealed to the underside of the
dash with lots of screws and a very feeble self adhesive foam seal. It also looks as if there were a couple of extra vents designed to supply air straight to the screen all the time. These are also partially sealed with the foam. I re-sealed with plenty of flexible sealant and metal tape.
Secondly where the plenum chamber matches up to the bulkhead air hole there was another badly fitted feeble foam seal. Took this off and replaced it with a thicker layer of denser self adhesive foam tape.
Now I just need to work out how to suck all the crap out from under the
dash. Mr Dyson is struggling.
Hopefully when it all goes back together I will be able to suck small animals and children off their feet when I turn on the blower!