Tony,
I think is the branch of the heating that he is drawing from, not the whole system run in 15mm. So there shouldn't be too many problems.
Agree it could be done quickly by more experienced people, but that doesn't mean that he shouldn't charge a decent amount to live off.
You have to carry a cetain amount of bluff. Your client won't want to think they have employed a greenhorn; just as much as they don't want to pay a fortune. Hence my question, what is the going rate for the area.
Remember you price has to cover your running costs for when you are not working. When you by a beer in a pub you are paying for the rent, electricity, bar maid, loos, peanuts
etc., Not just for the liquid in the glass.
I've stopped working out what I ACTUALLY earn an hour. Its too depressing. However, what I charge is another story.
Being a plumber is as much about running a business as it is joining pipes. Unfortunately.