The question is still very valid it's a big brother thing and although I could re-wire my mothers house and issue an Electrical Installation Certificate without a problem if it was Joe's house down the road would it lead to claims from the tax man?
The tax man would obviously only have any claim if you had been paid for the work. If Joe were a close friend of yours, it's far from impossible that you would do substantial work for him without charge, so the issue would be in trying to convince the tax man that you hadn't been paid (in cash or kind). An EIC would only indicate that you had done the work, not that you'd been paid for doing it - but I agree that if there was not even an EIC (or any other paperwork), it's unlikely that anyone (including the taxman) would even discover that you had done the work, whether you were paid or not.