The advantage of using a broker vs the lender direct is speed. You tell your broker a bunch of stuff once, if you want to change the mortgage halfway through buying you don't have the painstaking bank calls. First Direct made me lose the will to live. x2 45 minute calls, or five minutes on the phone to a broker. And they force you to listen to all their disclaimers (lenders), I bloody hated it. It also took ages for a decision (a week, instead of 12 hours), and appointments ((10 days time "so we can formally apply" - what? but you gave me a DIP you lunatic!!!)), instead of same day).
A broker will also tell your prospective lender's mortgage rate has gone down betweeen application and exchange, ask you if you want the new rate, get it for you, and have the paperwork reissued and it really is that hassle free. With a bank (at least with FD), they tell you you have to watch their rates yourself.
As for paid versus non paid broker: I have found them to be EXACTLY the same, and I have tried probably 9 of them.
Use a broker for speed. But, unless you have complex circumstances that need a lot of personal care and attention (where building a decades long relationship relieves stress and thus is worthwhile), I would not pay.
They could never get us a better rate than the non paid brokers anyway.
L&C (free) are fine. Sometimes they drop the ball, but frankly, they all do. They have also always had the best rates, after many years of extensively looking at all other options.