Southeastern ticket fine appeal, help!?

I have a southeastern season ticket, the other day i got on the train got to my stop and they were checking tickets. I reached in my bag and realized i had left my season ticket at home. I explained this to the ticket collector person and he gave me a penalty fare notice, he said i should appeal and send a photocopy of my ticket with the appeal. However i don't have a clue what to include in my appeal letter and any help would be great.