Jo wrote:

"I'm a skint student, can you please give me a discount?" and he said "If
you pay cash with no receipt we can take the VAT off".

It seems that you're the 'shifty' character - asking an honest tradesman
to cheat the system for your benefit.

No, I asked a tradesman to give me a discount. I didn't ask him to
cheat the system.

You wanted £20 out of the guys pocket?