About a year ago, I asked on uk.d-i-y if anyone could suggest a way to clean an enamel bath.
I tried various suggestions but none of them seemed to work very well.
Someone at work recently suggested that I try a product called "bath power". It's available from Sainsburys. It comes in a white trigger spray bottle. The actual trigger handle and nossle are red in colour.
The instructions say spray it on, leave it for 60 seconds and then wipe off.
I was very impressed. In one attempt, it stripped the dirt off and the bath looks as good as new. The bath has gone back to being completely white in colour.
I would thoroughly recommend it.
Graham