When they issue their Court claim, a copy is sent to you. If you dispute it (as you will) then you need to fill in the form saying you do not owe the money. You can then send your proofs (copies only - keep the originals) with the form. Make sure you send it off first class recorded delivery, or hand it in to the Court if it is local and get a receipt.
The Court then sends your response to the claimant. If the claimant does not withdraw, the case will go to Court and there will be a hearing, at which you can be present and make your case. The claimant has to provide evidence to prove to the Court's satisfaction that you do owe the money - otherwise they will not get a judgement in their favour.