Brokers can get better deals, they basically bulk buying mortage plans and if they refer enough customers to the same mortage provider a discount can occur even after the brokers commission.
The downside is that certain brokers only work with certain mortage providers or they will recommend providers that will give them the biggest commision, so it may not always be the cheapest around.
What your basically paying for is time.. it's part of their job to look at providers offering everyday so they should know which is the cheapest for the customer. It all depends how well a person knows how mortages work, how much time they are prepared to spend figuring out and how much they value their own time.
Given hindsight, I would have just paid for a broker myself. It's possible to pay a single fix fee for a broker advise for the whole of the term of a mortage. they may have advise to me to take the hit on early exiting of a mortage and to renewl before the rates hiked up. But hindsight is 20/20... and if they knew I should have known as well.
Any good mortage provider agent, will tell you the sums and which one of their particular deals is the cheapest, to pay a fee or not to, how much early repayments you can make... heck I've even had one hinted that I should shop around to find some deals on off set mortages and that they don't offer off set mortages themselves, and that I may find a cheaper deal as the lowest LTV amount they can do is 60% and it's possible to find ones at 50% and I had to just make a lump sum payment to qualify.