Dave,
I had this a few years ago, I had to go see my bank manager for an overdraft as I was owed loads.
He told me about the interest late payment charge, but he did say be cautious as if you bill them the charge , the company that owe, might look somewhere alse for their work to be done and then take the pee out of the next contractor aswell.
Lets face it we mean nothing to them and there will be many willing to take on the work.
I didn't charge the company owing me I just upped the phone calls to every day,weathered the storm and eventually got paid
Darren