On Friday, 23 March 2018 16:37:37 UTC, wrote:

Few diyers pay for the paper if they know what they're doing

...and that'd be the case for me normally. It's just that this time I have to involve BC for the extension (quite rightly IMHO) but I am worried that I won't get the completion certificate without the electrics being formally tested.

I might just change the design so that there's no electrical work whatsoever. The extension of the existing circuits could be done at a later date and, by virtue of them being non-notifiable, not require any sign off.