So if I've read this correctly then you haven't been stopped in person? Just contacted asking to speak to you regarding irregularities in your ticket purchasing history?
Normally with a situation like this a person would be invited to attend a voluntary interview, if you don't engage with them then as others have pointed out it makes it more difficult for TL to prove anything so they will have to investigate further by trawling CCTV etc, they may not find anything and leave it there or they may find enough evidence to proceed to court, or they may just take a chance and put what they do have before the court anyway and see how you react, they can always withdraw it at a later date, there are a lots of ifs and buts and the onus is on TL to prove your guilt. I'd say that if there are a large number of previous journeys that they are investigating then this could be quite big, in such a case only a qualified legal professional could give you the right advice so it may be worth considering