Last year, I had to get the people who put the kitchen, in becuase the the ink and taps were faulty. They replaced both with new work surfaces. They were contracted by the manufacturer. They were nice people, and they got on with the job and did it well. So I gave then a tenner to get themselves a drink.
Tne next day I picked up a document on the work surface and underneath was a bit of paper and a ten pound note. On the bit of paper was a note which said "Thanks for the tip, but we are not allowed to accept tips"

I think that bucks the system.