If I have understood the OP correctly, the position is as follows:
1) OP held a season ticket for their regular journey (let's call it stations A to B). On a number of occasions (on the basis of post #8 at least 32 occasions) their A-B journey (or the B-A return journey) was delayed by at least 30 minutes and so delay repay was claimed in the form of a Northern complementary ticket.
2) The season ticket expired. The OP continued travelling between A and B, using (post #34) tickets bought at the station or online, or on at least 32 occasions the complementary tickets already held. A number of these journeys resulted in further claims for delay repay. It seems that delay repay was claimed for some journeys made using the complementary tickets.
I'm sorry, but I'm going to sound like the railway here, and ask some questions where the answer might not show the OP in the best light. But that's so that we can understand quite what has happened, and so say whether we think that the railway has a leg to stand on in wanting to interview the OP and maybe take them to court.
a) Is what I have written above an accurate summary of what you (the OP) have told us? If so, then
b) when you were claiming delay repay in step (1), were all the claims for journeys at the same sort of time? So were you always claiming for trains leaving A for B at the start of the morning, or going from B to A at the end of the working day? If your pattern was different, is there a reason for that? I can see that you could have a reason for travelling at odd times - you might have variable shifts.
c) did you actually make the journeys from A to B (and back again) that you claimed delay repay for?
d) or did you obtain the complementary tickets from somewhere else (buying them from someone on the internet or a man in a pub or something: were they passed on to you by a friend or relative*)?
e) Looking at step (2), did you claim delay repay on any journeys that you made using a complementary ticket? If so, in what form did you claim the delay repay (money back, or another complementary ticket)?
f) when you were claiming delay repay in step (2), were all the claims for journeys at the same sort of time? So were you always claiming for trains leaving A for B at the start of the morning, or going from B to A at the end of the working day? If your pattern was different, is there a reason for that? I can see that you could have a reason for travelling at odd times - you might have variable shifts.
g) did you actually make the journeys from A to B (and back again) that you claimed delay repay for?
(Yes, I know questions (f) and (g) are almost the same as (b) and (c) - but I'm asking about different stages in the process, so the answers might be different.)
I'm asking all these questions because there are a number of points where the OP could have broken the rules: they might not have been entitled to the complementary tickets in the first place, or they may not have been able to claim delay repay in the second place. I do have to say that for a regular commute 32 complementary tickets seems to be a remarkably high number to have obtained so (in practice if not in law) there's something of a need for the OP to show why they had that many.
The OP also asks if there is any way they can get legal advice. The suggestion that someone has made upthread of asking if the OP is a member of a union is a good one - it's a quite frequent benefit of union membership. If that's not available, it's said that quite a few solicitors will offer an initial free consultation. But as far as I can make out, legal aid only cuts in for criminal cases if the case gets as far as being arrested or going to court.
https://www.gov.uk/legal-aid (and the following pages) have something to say about this - see what it says on
https://www.gov.uk/legal-aid/eligibility:
If the OP can't get legal advice and does go to interview, then they should at least ask whether they can bring someone with them. If the OP has a friend or relative who knows about the law, then so much the better - but just taking someone sensible who can take notes of what is said, and who can (if necessary) give moral and emotional support is a really good idea.
*I'm not sure if Northern Rail complementary tickets are transferable or not. Maybe someone here knows without us having to start a major search through the rules?