I know the law you can keep quoting different acts at me
That's because the SGA applied to the end of September 2015 and the CRA 2015 has applied since 1 Oct 2015.
The fact remains, a receipt is not necessary because it is not even a legal requirement to provide one.
I often go into M&S and get asked if I want the receipt. If I'm buying food items I don't bother. That does mean if I've paid cash I have no proof of purchase, but that's my choice. But if I paid by card, I would.
As for not getting prosecuted, Trading Standards have few resources and probably bigger fish to fry.
If someone has simply changed their mind, that's different (unless you buy online and return within 14 days).
I'm afraid that you are wrong on this. I am a lawyer and consumer law is one of my areas. And I dealt with this exact question this week at work - exactly - can a retailer ask for a receipt for faulty goods (outs me totally so namechanged).
As for the Xmas lights, presumably you take them out the packet, turn them on, they work, so you tell the customer that and send them on their way?
But if they don't work, you are going to still act as if they are trying it on?