Correct me if i'm wrong but you don't always necessarily need building control sign off. They could've used a contractor who is registered with a thing called the competent person scheme. Do some research and see? It could be waffle mind but I've known a few people do the same thing in the past.
Good to get some different opinions, thanks. I guess it boils down to whether we accept the risk or not... Why can't people just do things properly?!