I have a stain on my lounge ceiling that I have painted over but it just reappears.

Stains like this tend to be water stains or possum stains and are water soluble, meaning that they will keep bleeding through water-based paint.

Stains like this tend to be water stains or possum stains and are water soluble, meaning that they will keep bleeding through water-based paint. If the stain is small then you can buy a spray can of stain block to just treat the area or if that is not appropriate then brushable forms, like Solver Multiblock Stain Sealer, are available to apply to the affected areas.