Any question answered...

The ceramic under the waterline is brown, attacked it with bleach and a good strong brush and it's hardly done anything to it. The previous user didn't keep the best hygiene in work

That's limescale build up, you need acid to shift it, not alkali.

e.g. http://mistralie.co.uk/catalogue/product/97/Acidic-Toilet-Cleaner---Heavy-Duty---Premium-LOOBR

A sachet of kettle descaler normally does the trick. The brown stains are where the shite has bonded to limescale.

Can't you just pour a litre of coca cola down there and leave it overnight?