No idea if they are still competative, but when i bought new a year or so ago broadspeed's prices were about where the dealers would drop to, some quickly, some took more time, i looked at a few cars a qashqai was one of them, as they were selling so well at the time so it took a while to drop the price, in the end we decided the boot wasnt big enough so didnt buy in the end. However i think nissan have more competition in the market now (ford/vaux etc).
If you have a px have an idea as to what they sell for and what you can get a new car for and work out the difference, some dealers will offer you a huge discount for a new car but expect you to give your px for next to nothing and some may give you a great px value but drop nothing from the new car, ALWAYS look at the amount of money you are going to have to pay. That may sound obvious but some people get carried away with the the discount they are getting on a new car, or the high px value they are getting for the're old car.
So far as using a company that negotiates on your behalf i'm very cynical of paying somebody £100's to do something you can do yourself.