I have used a webiste called Rated People (www.ratedpeople.com) and found someone to do some wallpapering, plastering and painting. They ask for a price range you want to work within and then reccommend up to 3 tradespeople.
The guy who did the work for me, did it all to a reasonable standard for what I paid and finished everything within the timescale. I was also able to phone people who he'd done previous work for.
I've also gone with recommendations from friends, some have worked out, some haven't. Before I found the decorator from Rated People, I had someone come to estimate, who'd been highly recommended by a friend.
Anyway, he turned up with two other guys in tow and quoted �12K to do a bit of painting and decorating and install a new bathroom to my tiny one bedroom flat. After I told him he was a robber and told him where to get off, he said, ok he'd do it for �3K, So not all recommendations are best.