after trying lots of different diffuser materials, and positions I came to the conclusion that the best picture is obtained if the diffuser is as close to the disc as possible. also the thinner the diffuser the better, as less light seems to be lost. the problem is mounting the diffuser extremely close to the disc without it fouling (my disc is made of black card, and has got rather battered and wobbles a lot at low speed as it spins up until the centripetal force straightens it all out)
my sudden flash of inspiration, lightbulb style came when I looked at the scotch tape my mum passed me to stick some card or something on.
it's translucent, and a very good diffuser material! I realised that there's no reason that the difuser has to be fixed to the frame, and stuck small windows over the holes on the back side of the disk! the diffuser is now in the closest proximity possible to the disc, and there's nothing to line up!
My televisor is now fairly complete (I took it to a Meccano guild meeting today, as a couple of members had been interested when I'd told them about it) so I shall be posting some photos, and a descriptive page on my website in the near future.
I was going to keep a construction blog or something and post pictures here as I built it, but I never got round to it