Ecover multi surface cleaner, a fleece wipe ( normal fleece cut into squares ) and a little elbow grease. Spray it on a completely dry bath, leave for a couple of minutes and wipe. The fleece doesn't scratch the bath and attracts the dirt - I suppose a micro cloth would also do the trick!
I had also tried alsorts including the green bathroom cleaners, tesco green bathroom cleaner is alright but the ecover multi surface is definately the best of the bunch.
BDCS's other half