Fare evasion prosecution?

After completeing my train journey i discarded my single travel card (paid for in cash) as i was now able to use my oyster to finish my journey, however i was asked by a collections officer to provide my ticket (since discarded), now i have a letter from the prosections department!! am i now stuffed and can expect to have a conviction??? has anyone else had experience of the prosecution department for "fare evading"???