I once got asked to install a fused connection unit in an airing cupboard in a house, when I asked what it was for the guy said future use, then asked you will give a certificate for this won't you, I replied yes minor works, he replied not for the whole house then, uh no I said, I won't bother then he said goodbye.
I went to a house 6 months ago to do a periodic for a landlord, on arrival I noticed straight away everything looked new.
Landlord arrived and I said this looks new, he said yes your doing an Installation certificate for me, I said no you asked for periodic so its that or nothing, he said do the periodic then but no VAT, I said what, and he then explains he had a grant and the work is zero rated.
I made my excuses and left saying I would get back to him, never heard from him again so someone sorted it for him.