We used Lenny @ Hamilton Walters for my mums loft and they where brilliant. They were able to help her with the planning side of things as well which in the long run improved the end result as they were the ones who were doing the building themselves. Its ran really smoothly and my mum is a bit of a clean freak and she was so pleased with how they left the property at the end of each day.
Their rates are competitive and unlike all the bigger companies like Classic lofts etc, you are able to talk and deal direct with the owners which i think makes you feel more valued throughout. We did get a price from Classic as well, but it just seemed too generic and not personal enough for us!
Give them a call and you will see what I mean!
Hope this helps!
x good luck with it all! I know it can be a stressful time but it will all be worth it in the end. My mum is chuffed with her new room!