in a nutshell, first of all you need to write to them enclosing a copy of the outstanding invoice(s). You need to be firm and say that payment is overdue and if you do not receive payment within x (say 5 working days) then you will be forced to commence legal proceedings to recover the money. Send this letter Special Delivery.
After the 5 days are up, you need to write again this time informing them that since payment has not been received you will now be commencing legal action and that you will be also be seeking to recover costs. To avoid this action, payment must be received within 48 hours. You can also add interest as per the late charges
If they still haven't paid, unfortunately you must now commence legal proceedings. Depending upon the amount owed to you, you can file a claim at the Small Claims Court online.
If they don't pay they will end up with a CCJ and you can ask bailiffs to recover the money. Unfortunately this doesn't always work and you may not get your money back. You could also ask for the company to be wound up but you will incur additional costs for this.
