Look up your original documentation when you agreed to a monthly payback. The chances are it says something like 'reviewable after x months'.(posted 8443 days ago)It sounds like they are trying to see if your circumstances have changed...if you have more disposable cash then they want more of their money, and now.
If you're not better off now, it might be worth turning the tables on them and getting them to accept either a lower payment or ideally a lump sum (if you have one).
Don't supply them with any of the information requested... I can't see them taking you to court as you are currently paying them off 'open-ended' and the best a court could do is order you to pay them more money. Lets say the court added £30 onto their payment... how long would it take them to recover what they paid out in court fees etc etc.
DO, definately, go see your solicitor. Make sure it's somebody versed in this vague area of the law.
Finally, good luck and don't panic. You're doing fine.