Welcome to FT, mrsmagic147.

Well, you've left it a bit late I think. Four years ago this would have been much simpler! The two most common sites to track historical information are www.flightaware.com and www.flightstats.com Both of these will require you to sign up for searches beyond a week or so, and in your case I'm sure you'll have to pay a subscription to go that far back. The CAA also maintains these data, so you could have asked them to provide it if they were already involved in the dispute. Another thing you might try is www.oag.com, where you may get a free trial. The reason your flight doesn't show up when you search now is that it's a historic flight number and the older data are hidden behind a paywall.