District Auditors typically advise Regularisation via an EICR.
Part P registration may be a substantial business, with QA electricians che= cking work, but there have been instances where they are too overworked to = do it in full. Example being the quality of insulation work is often extrem= ely variable, the devil is in the details and workmanship can vary from per= fectly competent builders as "subs" to imbecile kitchen fitters not tighten= ing a screw.
Domestic electrician work breakdown structure is 99% perspiration, 1% calcu= lation, but being paid at rates for 99% calculation :-) So removing the per= spiration part tends to not go down well. That said, if an electrician will= not do it, then BC is the route, but beware they may want to see work unco= vered re cable sizes, spacing, routes, yadda yadda. The alternative is hiri= ng cable tracing tools from Fluke and so on. A load of bollocks considering= what I have uncovered over the years - many drama queen electricians were = atrocious for many years before achieving some level of competency and peop= le are having to pay twice: THAT is what annoys a lot of people about the e= lectrical system.