hi there
I've been involved in builders cleans for a good number of years.
yes its hard, health and safety is crucial, and all the other paperwork they require, but generally we do well out of them.
your key is a god relationship with all the important people, site manager, sales staff, and the QS.
at the end of the day they are generally buying on price, until you have a good relationship with the decision makers.
rgds
martin