To recap, last night I added 1 litre and went for a run.
Then this morning before going to work I checked the level and poured in some neat antifreeze (yesterday's fill was a bit more water than antifreeze). I only had about 300ml left and poured all that in and still it wasn't registering on the filler. I'm pretty sure I heard the 'trickling water' sound on the way to work.
At lunchtime I bought another bottle of anti-freeze and added about 800ml MORE down the filler, took the car for a 10 minute run and after that the level was OK (with car still hot). I will check it again tonight/tomorrow morning to see if I lost some more on my drive home!
Note that since I've been misreading the coolant level for ages, it hasn't been filled since my 30k service which was 3,500 miles ago. Still, that's 2 litres poured in out of a total capacity of 7.2 litres.
Firefox: I assumed the pipe from the top of filler pipe that goes into the wing was an overflow pipe that dumps excess coolant onto the ground. The pipe from the bottom of the filler goes straight across the top of the radiator into the header tank, so what do you reckon is in the wing?
Paul.