like ggirl. We are currently nearing the end of a loft conversion, and used recommendations of our structural engineer who turned out to be the retired chief structural engineer at the local council building control.
But to start with we called a few builders from searching the internet and got them around for a chat, to see how well they reacted to what we wanted, what they knew about building regs, how many corners they would cut to save money (ie we did not want them to), attitudes (ie would they do it our way, or their way or the quickest/cheapest way, and to hell with what we wanted).
Then we called up the local building control and gave them some names and asked if any of those we had shortlisted were known to them, and if they were cowboys. They could not recommend a builder, but they could tell us who to avoid with a bargepole.
We found a guy who felt right, and he recommended and independent structural engineer, who we really liked and who was the above mentioned. This guy, recommended 2 builders, one of which was the one we already liked.
It was a long and drawn out process and it took us the best part of 3 months, but the outcome (so far, got a week to go) was the right one, we got a good mix of competitive pricing and good workmanship.