Always beware of salesmen who are open to negotiation.
It usually means that the first quote was deliberately over the top and then they make reductions to make you think you're getting a bargain.
I was recently quoted �7300 for double glazing,which then got reduced to �4300 for exactly the same spec,and that's still over the top. Remember that the salesman has to make his commision and he'll make his money from you!
For a loft conversion of that size I would recommend at least 5 quotes from different sized companies,generally the bigger the firm/name the higher the price. A small local firm with a good reputation may well be your best bet.