In the UK some mortgage brokers don't charge their clients. I wonder, how exactly do they pay their own living costs? Is it true that banks pay them? But then, if a bank pays a broker, can I be sure that they will get me the best mortgage, better than I could find myself on money supermarket, or any other website that compares mortgages?
In the UK, mortgage brokers are either paid by fees (paid by you, the borrower) or commission (paid by the mortgage-lender) - and sometimes both. They should make it very clear to you upfront which arrangement they use and what the fees or commission are.
Obviously, if you pay their fee then there is no conflict of interest and you know that they are finding the best possible deal for you. However, you are paying for this service.
If they are paid commission instead then they might be finding you the best deal for them. This is a good option to take if you are a) very cash strapped and don't want to pay the fees, and b) don't mind about only getting a "good enough" deal, and not "the best" deal. They are incentivised to make sure that the deal they offer you is good enough for you to accept it of course, otherwise they don't get paid at all, but beyond that they will choose the best deal for them.
There is more info about this here, which expands a bit on what I have written above.