Step 1, send an invoice with clear payment terms.
Step 2, send one polite reminder maybe 7 days after the due date.
Step 3, send a Letter Before Action, with a further 7 day deadline (use a firm like thomashiggins.com to turn the legal wheels very cheaply)
Step 4, file a claim in the small claims court (again, thomashiggins.com are very good for this). It may take weeks but you can add interest and all the costs you've incurred.
The few times I've done this (as a consumer) the company has coughed up at some point just before or just after the court papers have gone in. For a truly hopelessly disorganised company this is the only escalation method that works.
The furthest it ever got was with Enterprise car rentals - I had a bailiff threaten to tow one of their vehicles before they would write a cheque.
This is all advice for uncontested debts - obviously if the company has a problem with the debt they may choose to represent themselves and argue the point, but if they were going to do that, they'd probably have engaged you first!