Hi again,
I have finally sorted out (thanks entirely to @Suphew) my 4ft light but the lighting is still really bland. I have a couple of new tubes. As I said earlier, I have two double lights like these: http://www.aquaone.co.uk/lighting_al_t8.php on my 4ft tank. One is 3ft, one is 4ft. The 3ft one has two tubes which say power-glo 30W and the 4 ft has one which says Aqua-glo 40W and one which says Sun-Glo 40W. Does that make any sense?
My tank is 65cm deep though so I am guessing I need some strong lighting to get good light. It is an african tank (mainly) with no plants, and mainly shell rock (is that what it is called?) like these ones - but a different tank.
(this is a photo of my old tank though, I have a background, etc etc now, and more rock, but it gives you the picture).
I am in the process of changing to black substrate (I was planning to buy some of that lovely jet-black silica from the LFS but if anyone can tell me a better place to buy it please do!). I have hauled all my substrate out today.
Anyway, so I would be really keen for any further opinions about lighting for a deepish tank, and also about substrate in an african tank.