I would write back and request a copy of the Money Judgement Order 'on which they are relying'.(posted 8163 days ago)Perhaps it is just another way they are trying to get you to pay up.
If they are not prepared to give you a copy simply ask why not.
If they have one then in the CPR rules there is text that says 'If an existing case was not put before a court between April 25th 1999 and April 26th, 2000, then it was stayed.'
Does this mean that if the MJO was before this, and therefore was existing court action, then they had to put the case before the court again between those dates. If they did not then you can argue that the case is stayed.