Christ, definitely at the higher end of what I was expecting. Thank you all for your replies, best get some more quotes lined up.
I would advise that you right down what you want priced and include some photos of your existing loft. It doesn't need to be too long ( 1-2 pages) but it will help the builders price on a like for like basis rather than them making their own assumptions. Remember to include a requirement for the work to comply with the Building Regulations. Builders will still need to visit to prove accurately but hopefully you'll be able to shortlist a few beforehand.