If you were unable to attend court then you should have informed the court who could have rearranged the date for you so you could attend. I would suggest you actually read what people have written as they are trying to help you, there was no administrative error as you believe and there was no 'fine' to be paid at that time as SET had not imposed one (nor do they have the power to do so). Your were found guilty of an offence and thus it was the court who imposed a fine.
Now you could try to appeal your conviction but as I said it would be heard in a crown court and you would need to provide evidence it was not your intention to avoid paying your fare when you were questioned by the RPI. You needto contact a solicitor, I'm sorry we don't know any magic loophole that will make this all go away. Perhaps if you took this a little more seriously before it went to court you would'nt be here crying foul.