28th. September 4pm because 27th. is a Sunday
A defence needs to address the issues and claims made in the original Notice to Keeper and the Particulars of Claim on the claim form. It sounds like you have the basis of a defence already - you paid, you have evidence of payment, their machines were faulty.
If you search the forum you will find that the majority of cases brought by Parking Eye and using DCB Legal are discontinued by the claimant before the court fee needs to be paid. So if you file a defence (simple, stick to the facts) and stay on top of the process with advice here you will very likely find this goes away after several months.
You have a court claim because the claimant knows that many people ignore them or miss the deadlines they impose, and can claim a judgment in default against you. If you research the topic by searching the forum for many similar cases you will get an idea of what to expect.