In my view a £20 tip is too much if it was done as a standard job during normal working hours.
£10 is quite sufficient.
Oddly perhaps, if I have a trainee with me the customers quite often give the trainee a tip, usually £5.
If I was to get a tip when I had a trainee then I would share it or give it to him
Tony
The tip was given to an honest plumber. Where I live they are hard to find so anything that keeps him in business is a plus for me. He could have tried to up the price a little but he did not. He quoted £85 and that is what he charged. I was very happy with the service.