Dave..... I had a VERY lucky escape from the very same chap you mention. It's a few years ago now though.
Some fantastic last minute advice came my way, in the nick of time.
Hugo, I'm sorry to hear your experience, and it's a shame you don't feel able to name them.
To all, be very careful that when you get a Plumber who purports to be Gas Safe, ask them why they don't have the actual Gas Safe Registration number on the sticker on the van. Some do, some don't..... but when I check those who don't have it, they are mysteriously missing from the Gas Safe register.