Maybe it's grime! The kids bath every night with bubble bath etc and tend to let the soap float around in the bath...it's a right pain to get it clean. I'm worried about damaging bath scrubbing with something like Cif but to be honest that doesn't budge it. I'm in the South if that makes any difference.
I ended up buying Sainsburys own brand limescale remover yesterday for £1.70 and whether or not the stubborn marks are limescale or not it shifted it. I thought that was quite a good price.