Visiting www.discovery2.co.uk, I just read a very good explanation of how to change the injector harness on a Disco 2. Very clear and concise instructions that anyone could follow.
As I understand it the oil gets to the ECU red socket via capillary action after passing the leaking O rings on the bottom connector of the injector harness. So replacement of the harness with a new one overcomes the failed O rings and cleaning the red plug and socket of the oily mess.
Now the bit I don't really follow in all the threads I have seen on this subject is, what about the wiring loom from the cylinder head to the red plug and the ECU that we have just cleaned, surely there will be oil on and in that loom too? Or am I over complicating it?

just clean both ends of main loom after changing injector loom ,you may need to clean red plug a few times but it will soon dry once fresh oil is not entering, it gets through by capiliary action before o rings

Since replacing my harness about 6 months ago I check the red connector every month and still get the odd drop coming out of the loom. It's getting less and less.

More to the point, the engine runs fine.

The trick is to keep checking the round plug on the front of the engine. As soon as oil appears there it is time for a new harness and you will have caught it before a great deal of oil has got into the loom.