I decided to have a go at clearing my scuttle drain today with the aid of x25xe's very helpful guide - cheers for that! To my surprise I discovered the cavity half full of water

Obviously it's been blocked for a while. I'm going to defend myself here and point out that I didn't realise there was a drain down there (I thought scuttle drain meant it's actually on the scuttle, like the driver's side one is) and being that my dual-fuel ECU is in front of the pollen filer I couldn't see anything without peering down with a torch.
If you let it get this bad it's loads more difficult cos all you can see is a big pool of muddy water, I had no idea where the drain was. A lot of poking around later (

) and the water started to trickle out from underneath the car, but it would promptly clog again. Eventually enough water drained that I could see all the crud and I gave it a good clear out then poured some more water down to check it drains freely.
Hopefully my windows won't steam up so easily now, time will tell!