Am I right in thinking N-Power are requesting a compliance certifiacte before they will connect new tails or something like that? I've had that from them before.
All I did was faxed them a business letterhead with my scheme registration number on it and simply put the details of the address in question and a statement which said something along the line of "This installation has been tested and inspected in accordance to BS7671"
And that was it.
Hope this helps