Hi Ben,
The lower hole Flossie is referring to is the wading plug hole, there is a second in the bell housing. Some people put a breather in the wading plug instead of the timing case FIP cover. But if you are going wading then fitting the wading plugs is a big bonus to stop the crap getting into the timing belt or clutch....
. Most remove them when not intending to wade, so that if an oil leak happens internally the clutch or timing belt whichever area has the issue is not contaminated. Wading plugs are chaep as chips to get.
The reason I asked was, I purchased a breather plate made from thin 1.5mm steel, ( so your 5 mm plate would not be a problem) , and the supplier tapped a hole in the ctr of the plate, which meant the pipe fitting would foul the FIP bolt.........So it never got fitted.
Cheers