if i was doing the job i would have said the i take all scrap that i remove if you want you take it out. You cant expect someone to take it out for you and you keep it so stop being tight or get your hands dirty.
not being nasty just truthful

He should not have taken them they way i see it we get around £100 scrap for a cylinder and pipe work in the cylinder cuboard....so with this in view your builder...was he a heating engineer? should be paying you back at least £100 plus interest.

Most trades take scrap that they remove...unless otherwise agreed beforehand.

This is something you should have discussed with him when you agreed on his price and hiring him.
From the onset, you should have made it clear you wanted all the scrap pipe.

The Builder clearly didn't know that you wanted to keep the scrap, or he wouldn't have taken it. He was only doing his job and tidying up after the work.
This is generally what happens.
Was this mentioned when he priced the job?!

It is the builders responsibility to clean the site and remove any waste at the end of the project. If you have not told the builder of your plans for the materials before hand, and so given him a chance to adjust his price for the project accordingly, then I think perhaps you need to reconsider your situation.
Regards
Mike.

Well we take all rubbish / scrap from site usually as customers expect us too however, i would expect to be asked to leave scrap if you wish to keep it as i have been done in past of course i feel thats not a problem, however unless asked i will clear up after myself.