I haven't used them (yet) but have you thought about using one of the 'trademen review' sites?
http://www.trustatrader.com/
http://www.ratedpeople.com/
http://www.trustmark.org.uk/
They aren't 100% transparent (because the tradesmen have to pay to be listed in the first place) but it might be a place to start.
Otherwise, there are a few tips in this link: http://www.money.co.uk/article/10091...dodgy-ones.htm