Brendank - great questions!
1. The screen looks good to my eye, though a pure black one would have been better. Definately better than a colour in my opinion. The side walls are in keeping with the upper wall and ceiling colour (Kodak 18% grey for neutral light reflection on the screen). There are some pics on my website if you've not seen them. I can do more if you like.
2. Being a loft, it does get approx 3 degrees hotter than the landing below, but that's because of the sun on the south facing part of the roof. I don't think the carpet makes a great deal of difference insulation wise, as it gets cold in winter too.
3. I fitted a sliding vent into the ceiling and the velux window also has vents. I was going to use remnant of the old brick kitchen cooker chimney as an extract outlet to aid ventilation, but I haven't needed to yet. It doesn't get stuffy up there at all. I think the cross ventilation within the roof helps this, and I'm going to fit a filter onto the sliding vent to reduce dust ingress.
4. I'v only installed 3 halogen 50w (240v) spots into the ceiling which I bought from Argos. Along with a remote dimmer, it gives the room enough light to do most things, and looks cosy at the same time.
5. I would either install reflective material into the roof prior to fitting the plasterboard, or easier still, use the plasterboard that comes with it built in. That would reduce the heat gain in the summer a little.
I may change the screen wall carpet for pure black one day.
I might get the floor up and have a floating one fitted with steal beams so that I can get the purlin supports removed.
I will cover the purlin supports with black carpet or felt instead of repainting them black again.
That's all that comes to mind at the moment..
Gary.