If you have been summoned to court (not just threatened with court action) the priority is to get the court case stopped. You have good grounds to do this as HMRC should not be pursuing overpayments through the courts unless you have been completely ignoring their letters.
The first thing to do is contact your MP as soon as possible. If s/he needs persuading that s/he needs to act immediately (i.e. now) insist s/he is your only representation in this matter and it is her/his duty to uphold your rights and s/he is the only one who can contact the following offices on your behalf. Your MP’s phone number should be in your local telephone directory and they most likely have a website too. If you're not sure who they are, click here to search for your MP's details. Their office and staff should be available weekdays during office hours.
Although things have improved very slightly since the very early days, please don't make the mistake of thinking that if you get to court you will automatically have your say! HMRC can issue a 'Certificate of Debt', and the court normally HAS to find in their favour. It is quite possible that nothing you say will make ANY difference. Read A Cautionary Tale, if you haven't already!
Also, if you have received a summons but haven't started the dispute process yet, it might help you to read Dispute Not Started?
Now go to How to Dispute and get yourself started. You may have done some of the steps, i.e. the Data Request (Dispute Step 1b) and/or Contacting Your MP (Dispute Step 3). That's not a problem. Just make sure to do the in-between bits.