Notice too, that there is no time limit set out in any of the text quoted by Specs. I've heard some say that you can't be prosecuted if more than 6 months have passed, but I would not like to test out that theory.
Over 4 years of being self employed now, i have had about 4 or 5 phone calls from people who've had electrical work done and now want to sell their house and so "need a certificate". in a couple of cases, the owners were pleading with me to "help them out" as they were only a few days from exchanging and it was going to fall through otherwise. i know that one sale did fall through.
I rather imagine that at the 'inspection' stage someone has picked up that there has been some work and the purchasers solicitor has sussed that there is no appropriate 'paperwork'. He then, of course, advises his client not to exchange until the issue is put right. Then the panic sets in.
In these cases I will offer a PIR, but tbh I prefer not to get involved.