Thanks CM!
Im just in the process of having mine finished. Ours was started 9th dec so it has been coooooold having it done this time of year. However, this time of year is usually a quiet time for this kind of work (xmas & winter) so we managed to bargain on the price. We also bluffed a bit and played one company off another and ended up with a total bargain with 23k. (other quotes ranged from 27.5 to 37k - we are in North London)
We used a 'specialist' loft company, and DP used his work's credit check company to see if they had any ccj's, check their building certification etc etc. They came up good so we went with them.
It hasnt been plain sailing, and the mess has been horrendous. The 50 years of soot and crap from the loft as it was being pulled down, the scaffold around the house for 6 weeks just seems to make windows even more dirty but cant be washed, and the dust from the plastering - well - its like having a toddler let loose with a dozen bottles of talc! AND it forms a paste when you try and mop it up. Not fun with 2 kids under 3.
There are still odd jobs to be completed, but we are holding on to final payment (we have paid in stages as each stage completed) until the council building inspector signs it off. (You wont be able to sell your house in future if its not signed off).
With regard to architects - the company had their own architect as such, but ours was a straightforward conversion with no special considerations. We didnt need planning permission because you are permitted a certain amount of extension/conversion - i think 40 or 60 cubic metres before you need planning.
We have ended up with a good sized third bedroom now (5.5m x 3.5m) plus a 2m x 3m room which we will kit out as an ensuite in the future (the pipes and plumbing are in place and capped under floor ready). For the time being its an office.
Despite everything, its been totally worth it, cheaper than moving i believe and has undoubtedly added value to the house to at least the cost of the conversion.
HTH.