I am not a lawyer, but surely it's not just the ticket buying but also the ticket use and the ticket refunding?
In this case, the OP is very clear that they have not tried to refund tickets that have been used, and it's my impression that they are clear on the distinction between 'used but not checked' and 'not used so not checked ' (to say nothing of 'used and checked'). So should the matter reach court, it's hard to see how a prosecution could succeed. As I understand it, the purpose of this thread is to help the OP marshal their evidence such as to get the railway to discontinue their check as soon as possible, and so stop taking up the OP's time.