London Transport are taking me to court... I need help!?

I think this country just wants to make money out of it's people you know. Let me tell you I have never in my life gone without a ticket on public transport. I have lost tickets but I'm never bothered. Now I'm bothered. I lost my ticket on board a bus some how and got inspected and then received a letter asking me to come to court. You would have thought there'd be a fine on the spot but instead I was asked to find my ticket where ever it was in my bags or what (by the inspector) and have it ready till I received a letter to my home address (that was to send the ticket with the letter to show I paid my fare). I didn't find the ticket and I ume now some one on the bus took/found my ticket and used it for themselves, when they were asked to show their ticket. I had made it clear in the comments bit for the first letter that i touched my oyster card which i found had no money, asked the bus driver how much the fare was, paid him, nearly forgot to take my ticket which I got reminded to take, took my ticket, sat down with my bags, saw the inspector board, started looking for ticket and then it was nowhere to be found. I even missed my stop when the inspector took my details down and he even asked me for my ID which I showed him to clarify who i was. I also mentioned in this letter that still today I have not found my ticket. Anyway now they have sent me another letter asking for me to either plea guilty (for not having a ticket) by post or to plea guilty or not guilty at court. I really don't want to plea guilty for not having a ticket because I know that I paid. I just wonder why didn't the inspector ask the bus driver about me??? It was just minutes after that bought a ticket and sat down and why don't they bother to check the damn bus cameras??? There's like so many of them as well. All the evidence is there that I paid. I just feel so upset and top of that I'm a student nurse and this is a conviction or some sort, so it's going on my criminal record if I'm right?? What should I do??? Shall I just pay the fine and plea guilty then????