Since we’re at the start of our renovations of our house, here’s some advice taken from http://www.how2pull.co.uk/

Always get at least three quotes and insist on paying against specific jobs completed to your satisfaction. [Yup, we did this]
Always check past workmanship. [We phoned previous clients]
Trusting a single-line quote is a recipe for disaster. Quotes should be detailed. Break the job into logical components. [We were told to be aware of this from our friends]