I'm going to be moving into a shared flat where there'll be one tv that we'll only use to watch dvds on. Should I tell the tvla and ask for a declaration of not needing a license, or is it best just to leave it?
Do not contact them.
I cannot stress this enough. By contacting them you are just providing them with a direct correspondance and they may attempt to send an inspector round to get into your flat.
You do not need a licence and are under no legal obligation to inform them of this.
Its like if you dont have a car.. do you call up the DVLA to inform them that you wont be requiring a tax disc each year?