These guys (I had a recommendation on this site for them) did an absolutely fab job for me - replaced both the door and door frame and did a glass door for less than the London Door Company quoted for the door itself, and with Banham locks instead of cheaper ones. I get a lot of compliments on my door - please PM me if you would like a photo!
They are great all-round builders and since the door, they have also done my bathroom and a loft renovation!
Ernest and Sergy
Also check out their recommendations on my builder