So I removed the scuttle panel earlier to have a closer look.
There were no blockages/leaves from what I saw and water was draining off fine through the hole and down the side of the passenger wing.
However I did notice that water was seeping in through the plastic clips that mount the scuttle panel to the body with screws. The rubber underneath some of the plastic clips has perished and was letting water in.
I could see the water dripping down looking right up in to the blower motor box.
Cleaned all the surfaces down and removed the shot rubber, then went on the attack with the waterproof outdoor sealant.
Hoping this does the trick.
I'll do more water tests tomorrow as the sealant should have set by then.
I was watching this video last night funnily, which also helped to diagnose the issue: