Don't let them take the piss mate. Don't quit and stand your ground. If you are ill, you are ill. They can threaten you with disciplinaries and what have you but if your doctor can write a note just so you've got some proof there isn't much they can do. I would also make a complaint about their behaviour - if I remember rightly, this isn't the first time you've complained about your managers doing this sort of thing, is it?