If you booked through an agent - then to them.
If you booked direct - then get the details from the County Court.
Making a Claim You can download leaflets that tell you the procedure.
If you make a CLEAR and DETERMINED demonstration that you WILL take them to court - they will almost certainly settle out of court. So don't pussy foot. Threaten them and tell what you will do and give them a time to respond, using recorded delivery.