I am going to tackle the living room nicotine stained ceiling, do I have to wash it down or is there a good emulsion which will not let the staining come through.
I second Ummmm's answer. You don't want to be painting onto nicotine stained surfaces. They are highly sticky and you're going to end up with a right mess on your hands.
Your finish will only be as good as your preparation .. just over-painting will probably allow the nicotine to bleed through again and it will be looking brown again before you know it.
As previously stated ... "sugar soap first".
If its painted with matt emulsion you're going to struggle to wash it down properly. In my experience you'd be better washing it down with sugar soap, give it a coat of Zinzeer water based stain block, then painting it as normal. In the past I've used masonry paint for the first coat as this can act as a stain blocker.