I was actually surprised how little coolant I lost when I took off the housing. It's probably taken about 2L. I thought about it and the chances of the thermostat AND matrix being broken are surely pretty slim and seeing as the thermostat was definitely nackered so I'll try and bleed it again later on today. I'll take the cap off and just leave it until it's really cooking and with any luck a sh!teload of air will come out
If you only put 2L back in/out, doesn't sound all off it came out and possibly a nasty airlock somewhere in the system.
As you have fitted your new theromstat, I would do a hardcore coolant change

. What I mean by that is, run the car to 90c, switch off and drain whilst the thermo is open. Let the car cool for a good few hours and then fill back up.
Obiously at 90c it will burn your hands off, so use thick gloves and be very carefull. I would expect alot of folks are against what i have stated, but in past expierence, I usually get most of the coolant out of system this way.