Mike Harrison wrote:

If, for example, my Grandfather were to sell next year and the buyers (or
buyers solicitor) pressed for certification of the electrical work done,
what should he do?

Disconnect the feed at the house CU. Tell buyers it is their responsibility to inspect/certify etc.
if they reconnect.

I think it's highly unlikely anyone's going to focus on the electrical
aspects of the garage; he can prove he's got building regs approval and
planning permission and that should be the end of it. But of any
particularly anally retentive solicitor does home in on it, he should
just shrug his shoulders and say he hasn't got it; so what? Are the
buyers really going to pull out of the sale because it's missing,
particularly when there's no evidence that the electrical work's not
properly done?

David