My gut feeling is that the law doesn't really achieve much, past a level of discouragement via FUD, since it's pretty well unenforceable. Electricians are supposed to write out "compliance certificates" for any work done, but I gather an awful lot don't. Even then, unless the work impacts the external connection, the only copy of the certificate resides with the home owner. I imagine an awful lot of people file it under the sofa, or in the garbage bin. As far as I know, there is no central record of a house's wiring configuration. Practically, the only way of telling that a job wasn't done by an electrician is that the job is really bad (and dangerous), or of a standard that significantly exceeds that achieved by a typical electrician.