Oops...

Question - what happens when you leave R alone with:

a) his new camera
b) the diagram below
c) this website http://www.electricstuff.co.uk/extube.html#focus
d) a smal spec of dirt on his focusing screen he cannot quite reach

Well, it is very simple, involves most of R's smaller tools and looses him an hour of his life. However, after several tries (the 400D is quite different to the 300D mentioned in the article) I managed to get it all back together again - and luckily the spot of dirt has gone.

Phew!