Independent (check they truely are) mortgage advisors are definitely worth it. Look for one who claims to be "whole of market" - you don't want one who only works/sell a limited selection. Many estate agent ones are not whole of market - beware!
They definitely can get better deals. Some mortgage companies (like platform www.platform.co.uk/ for example, the co-op mortgage arm) will only deal via intermediarys, and will not allow you to apply directly.
They are often cheaper, as they know that the paperwork they will get has already been checked and should be correct. It's more efficient for them, you get a discount.
In terms of paperwork, it's amazing how much a decent mortgage advisor will do for you. It's also amazing how much they go "oh, dont worry about that, they won't care for you". That insider knowledge is really valuable.
Even if you end up with a big highstreet lender - some still offer better rates.
No, they aren't all free - but you'll often save the couple of hundred quid in fees within months of the mortgage. Also, those fees are not set in stone - negotiate. They'll get commission as well (and they should tell you what that is) so if your case is simple they will often be flexible on that fee in my experience. Always ask!
I've used (and have friends who have used) Mark Humphreys
www.mhcmortgages.co.uk/contact
I'd not hesitate to recommend - and given he does everything online I don't think location would matter (used his recommended solicitors as well, who were superb if you want to work online and via email - I did!)