Hi, you've probably already sorted this but I had the same issue with the flat I rented to two girls! The entire bathroom was thick in mould, as was my bedroom wall as they'd kept a house cat in the property and never therefore opened windows or doors to ventilate it. I bought a proper mould remover, sprayed it over and over..left it and wiped it off. It honestly wasn't too bad and honestly looked like it would of been horrendous.
Good loo though, not nice at all