I would suggest as soon as you get the letter you call them and plead your case over the phone and offer them an out of court payment. Hopefully they will go for this and that will be the end of it. Alternatively there are plenty of specialist solicitors out there who will be able to help you, it all depends on how much you wish to pay against the risk of a criminal record for 5 years? I had a similar situation on a No.25 bus a few years ago, I was 60p short on my PAYG oyster and honestly just did not realise. I got the prosecution letter, I argued that I was not allowed to leave the bus by the inspector which is holding against will, that he had not read me my rights so nothing he said would be admissible in court, and that i had offered to pay the fare or the fine right there, and as the law states for Fare Evasion, they have to prove that you did not have the means to pay the fare at the time you were caught. I received a letter back stating that there was no record of this incident and so to ignore the previous communication. So, as I say it's up to you, it all depends on how you worried you are, how much money you have to spare and if you're willing to fight it on the basis you believe you are in the right and East Dulwich Stations ticket buying situation is an absolute joke at the best of times! Good luck either way!