Firstly, has the claimant (creditor) followed the proper procedures and attempted to negotiate out of court? See "collecting a debt".
A General Procedure Claim should be a last resort for the claimant. It is a court action to recover debt heard before the Magistrates' Court.
1. You should seek legal aid and get advice from your local community legal centre. The legal centre should also be able to help you apply for legal aid and also offer financial advice.
2. Work out if you accept the debt or are disputing the whole or any part of it.
3. Respond to the General Procedure Claim within the time frame given. This time frame should be contained in the claim itself.
4. Follow the steps here.
5. Throughout this whole process, try and contact the claimant and come to a payment arrangement after explaining to them your financial situation. Confirm the arrangement and conversation in writing via email, letter or personal note. Record any subsequent conversations (and attempts to contact the claimant if they do not answer) in writing, including date, time, contents of the conversation, to whom you spoke with etc.
If the claimant is a financial or credit agency (company), contact the Financial Ombudsman Service and see if they are able to help you. Generally, where the debtor is in financial difficulty or the creditor has not followed proper procedures in attempting to collect the debt, a claim or default notice or similar action should not be initiated.