Hoping Nige has some B&B related expertise.
Sorry for the delay matey, been away for short break in Yorkshire.
Anyhow...
Limescale is the swine of the bogs. Beat that and there's not much else to worry about (unless the enamel is knackered and its new bog time)
Try some Harpic 100% Limescale Remover. It may need a few applications (ie using a reasonable brush also) before you get results.
Failing that, a reasonable new bog would probably set you back around £60, probably less if you were just replacing the pan and a direct replacement was still available.
Billy Bog Brush.
No turd too clingy, No seat too mingy.