Once you have defined the scope you will need to choose a builder that has experience of your requirements and has done similar work. You will need to check they have insurance.
You should ask for references and follow them up. You should always be very wary of trades people that call unexpectedly - check them out. Look at their previous woork, take references.
works completed April 2006
The Study Prep School, Wimbledon - www.thestudyprep.co.uk On going general building / refurbishment during school holidays over the last 15 years
SmithKline Beecham - Contracting for 10 years Walton Oaks, Burgh Heath, Brockham Park, Reigate - General building /refurbishment, laboratories, buildings, offices, etc. SKB now relocated and centralised in Harlow, Essex