There are two things that you need to know:
1) If you ignore this letter, then it is overwhelmingly likely that you will be taken to court. So to have any chance of resolving this out of court, you need to reply.
2) TfL are known to follow people who they think are fare-dodging. So if you have used your mum's freedom pass on other occasions, TfL might know that already.
And there's something else that is useful to know:
3) TfL rarely agree to settle out of court, but have in recent months done so in a few cases.
So my advice is that you should reply to this letter. You are entitled not to incriminate yourself, so tell them what happened on 8 March and don't talk about any other occasions. In particular, if you
have done this before, you should not say that you haven't, because TfL may well know the truth already - and if they catch you out lying on one point, why should they believe anything else you say?
@Hadders has some excellent advice on what to put: they will probably be along to add it here, but otherwise have a look at
https://www.railforums.co.uk/thread...ngly-possible-prosecution.244466/post-6109585 which will give you some idea of what you might want to say.
But bear in mind my point (3) above: TfL normally prosecute, so there may be no way to avoid ending up in court. You will need to work particularly hard to try to get them to agree to settle this matter out of court - and while that will avoid a conviction it will still cost you money.
