I re-mortgaged about 3 months ago and am now with the Newcastle Building Society, whom I had never heard of until I started shopping around for a new mortgage. I wanted a mortgage which was relatively flexible, a good rate and no high charges (fair nuff? actually really hard to find but the Newcastle came out top when I was looking).
It is worth checking out the best buys in the weekend newspaper, e.g. Sunday Times, Mail on Sunday etc, as I didn't find I could get the whole picture looking just online with comparison sites. Once you've found a deal you think is interesting, check it out fully, with all the small print and put together a list of its pros, cons and charges and - critically, TRUE COST (over, say 10 years) - and you'll be able to do your own analysis and see what's best for you.
I did think about going to see an IFA or a mortgage broker but the once I telephoned first off took about two weeks to get back to me and by then I'd found the mortgage I wanted myself. This might be a good option to help with the search too. There is also What Mortgage magazine, but it seems more useful with info for people who are new to mortgages rather than if you've been through the mortgage maze already. The newspaper are most up to date with the latest deals.