Hi John
You might try Vanisol toilet cleaner it's a high acid toilet cleaner, you have to empty the pan of water and apply the cleaner with it's own small mop type applicator and leave for a few minutes for the contact time to work, I have used it on toilets that had been neglected for years and it brought them back . I dont know where you are try the sponsors of this forum or Smith & Osman in Cowley they have it.
Patrick