Thanks for the help. It turns out that it wasn't a fault with the computer or the projector, but with a cable fault in the ceiling linking the two. This had previously been ok but had been modified in my absence. So no problems with Leopard.
Thanks specifically to ElDiabloConCaca for taking the time to post a reply. You are completely correct in what you said and I'll know better if there is a second time.
Cheers