Im guessing your not actually posting directly from the job now so;
Send a letter and attached invoice, stating that it is overdue, they have 7 days to pay.
Explain in the letter you will be looking to claim interest from the due date at 8% per day, every day until it is paid. (8% is what you can claim in the county court - business to business)
And also that if payment is not recieved you will not hesitate to pursue the money owed via the county court. You could also include the draft copy of the court claim if you want to really drive the point home.
Dont do what I did a few months ago and just wait and allow yourself to be stalled by the debtor, get straight in there, and send all correspondance in writing and via recorded delivery (signed for).
And as daft as it sounds, if they offer to pay by cheque because they dont have the cash (trying to fob you off) take the cheque and bank it, even if you know it will bounce, it demonstrates that they accept they owe you the debt, and also gives you an easy win in court, the bill of exchange act means you cannot bounce or cancell a cheque for payment.
I've just been through all this, and let the customer fob me off for at least a month, coming up with stupid reasons for not paying claiming she had issues with the workmanship, I kept going back to rectify the "faults" and reached a point where she couldnt come up with any more excuses and just cancelled the cheque.
I've just obtained a CCJ and court warrant of enforcement to recover the cash, via the county court (money claim online) via a default judgement becuase she issued then cancelled a cheque, there is no defence to this is work is done, [DLMURL="http://www.easylawyers.co.uk/debt-recovery.php"]See here[/DLMURL].
She knew she couldnt win and just ignored the court letters.
I should in hindsight have taken action straight away, but didnt, so three months later im still chasing the cash..... or should I say, from next week the court baliff will be.
PM me if you like, i've become quite well versed in the county court process latley.
How much are you owed?