I clean our toilet using a spray with half vinegar/half water. I spray it and leave it for a while before cleaning with the brush. This is satisfactory. However, can anyone tell me how long to leave it, or indeed whether it has to be left for a while at all? Can you just spray it and brush it away immediately?

I guess this depends how often you clean your toilet. It definitely won't hurt to leave it overnight to soak once in a while.