If I'm reading this right they are sending enforcement letters to another party eg your relative and don't have your name or address? If so first step is for your relly to get in touch with TFL and say not me guv.
Take it from there. They may threaten enforcement against the person for letting you use their card or they may somehow establish you as the person who evaded a fare, in which case then yeah deal with it. Or they might do fuck all, which I would have thought fairly likely given you fare dodged £3.60 not thousands and thousands over a prolonged period, in which case sound.
I mean i'd agree with seeking legal advice if they were writing to you but as far as I can see they're not?