I am assuming that the debt is the same one that you had last September? If so, what payment arrangement did you eventually settle at?
http://www.consumeractiongroup.co.uk/forum/showthread.php?452222-Court-Enforcement-Services-threatening-to-visit&p=4792070#post4792070
If you have been paying without a missed payment since setting up the payment arrears, then you must speak to the company this morning to ascertain where the problem has stemmed from.
You also need to find out what additional fees may have been to your account.
Please post back once you have spoken t