I've run through the procedure of looking at removing heat from under the bonnet with my V12 Jaguar XJ-S - it's a much bigger problem than a Disco :-)

You need to be scientific about it:

First off - do the "wool tuft" test - stick lots of litt bits of wool on the bonnet about 2" long. When you drive a various speed you will see the centre of pressure move forwards and backward along the bonnet. At low speed, you may find a negative pressure just behind the radiator, and at high speed a high pressure at the base of the windscreen.

Next to to buy a Dwyer Magnehelic gauge- it measures differential pressure - it is very sensitive used in the A/C industry.

Use it to measure the differential pressure to see if you actually have a problem.

Here's a few articles.

Here is an article on Autospeed:
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Iain
300Tdi Brisbane