Legally, Mrs Overhang, the builder is responsible for complying with the Regs.
There should have been several stage visits.
Phone local district council and ask for Building Control, and explain the situation.
If the builder is being a div (quite likely), then for your peace of mind, Building Control might arrange to carry out retrospective inspections.
There was a time when some of them didn't issue a Completion Certificate, but I think I'm safe in saying that sense has prevailed and they all do now.