How late is the payment, Dave?
The amount isnt important. But the overdue date is. Best practise is; Go and visit unannounced. It doesn't matter who you meet initially, just get in to speak to the right people politely. If they're not there then you go back, it depends on how big the debt, but you go back personally, and promptly, you may be getting in before the receivers do.
Always politely, with all your paperwork intact, seek the person who issues the purchase order, seek the individual who authorises the work, seek the person who pays the cheque. Follow this as far as you possibly can personally before you seek any other help. Don't be fooled into thinking that paid legal or strong arm help wil be more diligent than you in seeking your rightful payment. They too only want your money.
I have only lost £400 in 25 years.
