You go to the customer service desk, say "This doughnut is stale" show your receipt along with the partly eaten doughnut, then get a full refund or exchange as you prefer.
If you're doing it daily the rules will change. :)
> this donut's a bit stale, I'm not prepared to pay anything for it thanks Mr.Tesco.
This is irrelevant. See quote from comment you're replying to.
> Sometimes you get a shitty waiter, that's life, but now you get to unilaterally decide you'll get that service for free?
No, the waiter still gets paid.
I've gotten burned a few times when buying bags of things like potatoes and onions and not seeing a bad one in the middle that had started to infect the others. Never had a problem returning. Aldi even gives money back plus a new item
What shop do you know of where they'll let you choose food, take a bite and then pay what you feel it's worth? Hmm, this donut's a bit stale, I'm not prepared to pay anything for it thanks Mr.Tesco.
Sometimes I buy fruit and it goes bad too fast. That's life. Sometimes you get a shitty waiter, that's life, but now you get to unilaterally decide you'll get that service for free? Because there's a significant minority of people who will always find fault just so that they don't have to pay.