If its a ltd co and they owe over £750 you can actually instigate a Winding up process through companies house, you usually find that this gets them listening and taking you seriously and sorts the issues pretty quickly.
Transcript taken from compulsory liquidation
This procedure is initiated by an individual creditor who must be owed at least £750. Occasionally a group of creditors acting together may take this route. The mechanics involve the serving of a Winding Up Petition on the Company, which has a number of draconian effects on the future conduct of its affairs. In particular, if the Petition is accepted by the Court and a Winding Up Order made against the Company, not only does this put it into Liquidation, but it makes each and every one of it transactions since the date when the original Petition was served potentially invalid.
Once the Winding Up Order is made, the Official Receiver automatically becomes the Liquidator, but he will often have a licensed Insolvency Practitioner appointed in his place, either where there are assets to be sold and a potential surplus for the ordinary creditors, or else if the creditor who served the original Petition or some other creditor asks him to do so.