They say that you have made an excessive amount of refund requests, and that they are potentially fraudulent. So things to think about (but not necessarily answer here) - how many refunds have you requested and why did you request those refunds? Have you been entitled to the refunds you've made or have you refunded tickets you used?
This is good advice. If all of your claims for refunds are legitimate (you bought the ticket but you didn't make the journey - it's not a matter of whether your ticket was checked but whether you used it) then tell the railway that. Challenge them to present their evidence of why they think you have been defrauding them.
If on the other hand you have been making refund claims that you shouldn't, then you need to be looking to see if you can negotiate the best way to resolve this. Work out what refund claims you have made that were legitimate and which ones weren't. Work out how much money you have had back that you shouldn't. That will tell you the very least that the railway will be wanting - and they will also want the cost of their investigation.
As the email suggests, the railway have details of what refund claims you have made, although they don't know why you made the claims. So while you don't need to incriminate yourself, don't try to say something that the railway know isn't true.
If I understand you correctly, you have already emailed the railway. So wait for them to get back to you (which could take some weeks or a few months) and depending on what they say, think about what I have said here.
