Oh I should have mentioned I took those photos while mine was in pieces! Dezzy's pictures earlier on in the thread show it with everything assembled.
From the factory that hole is covered with fabric tape then painted over so its quite hard to see. Removing the lower windscreen trim will definitely help. Be gentle with it as by now the plastic can be quite fragile.
The drain cleaner is for the scuttle / rain tray drains which are underneath the outside portion of the heater box. They're impossible to get to with the car in one piece so drain cleaner should sink down in to them and dissolve all the leaves and crud which blocks them up. Its a bit harsh though. No idea what it would do to the paintwork, or the aluminium body and gearbox casing it will drain on to.
The orange doodad here is one of the two internal drains:
You have to remove the lower dash panel, the footrest, and the glovebox to get the console side panels off (probably a diy on here somewhere), then the drains can be removed and cleaned. They go through the sound deadening and in to the tunnel where they drain out behind the tunnel heat shield. They pull out easily enough but its a bit of a fiddle to get them back in. Seat them in the tunnel metalwork first, then connect back to the heater box.