How to get money out of an agency that claims to have 'problems'

[TykeMerc]

Thanks to everyone who provided advice.
When I mentioned the late payment charges and that I would get in touch with a debt collection agency, the problem suddenly disappeared.
Funny that...

Indeed it's amazing how these issues can disappear when you react in just the right way.
Well played if it's your first time with this situation. Sad to say it's likely not to be the last.