It is more likely to be housing association than council i would think. Even if it is owned privately, the council could be paying the rent for the tennants.
nusiance neighbours probably will not decrease the value of your property per se, but may well make it harder to sell (meaning you might have to lower the price to sell it) of course this only applies if you are selling the house now - if you are not selling then anything could happen before then (ie they could move out).
As to what you can do there are a few things - firstly if they are doing anything illegal you can report them to the police. If they are making a lot of noise you can report them to environmental health at the council. If they are doing things that are a health hazzard, you can report them to the council (they don't have to be council tennants for you to do this)
if you can find out the landlord, then you can open a dialogue with him/her