Scott,
Once the work is complete, then the invoice is due. Giving someone 30 days or more has no basis in law.
Generally though I work on strictly 30 days. After this a 7 day payment due letter is sent, stating "no further reminders will be sent and legal action will be taken".
After that I goto moneyclaim online and issue a summons. Ther summons usually makes them pay, with all sorts of excuses why they forgot.
Even after that, if they call for more work to be done, I tell them on a pro-forma invoice only.
Remember business is tough, and its not until you go through the pain of waiting for money, to keep going, you realise that you yourself have go to act.
Don't Mess About When You Are Owed Money, It Could Send You To The Wall.
CATMAN