You'll be lucky. If you took FRI3 of £80,000 or £100,000 (SO2) in the last 2 years or so you need to read the small print of what you signed carefully.
The FRI3 paperwork states clearly that by accepting the FRI you rescind the right to PVR for the 5 years ROS.
When they retired my ac type last year I asked manning very nicely if they would consider releasing me under the exceptional circumstances clause and pay back the FRI. The answer was a categoric NO. The statement from manning was that no-one would be released from ROS on FRIs or other ROS under any circumstances bar extreme cases. Just wanting to change ones mind was not justification.
I do not know if you would have any legal case. That route did not appeal to me as a little word in my personal lexicon called integrity meant that I accept their decision as I signed the paperwork accepting the FRI and its associated conditions and I will abide by my word. Before anyone bites and says "but you did try to get out of it", yes I did, but I did it politely by asking if it were possible, being fully aware that the likely answer would be no, and prepared to abide by the decision. I also did it with the full support of my command chain and deskie, as at that time they did not have a post for me to fill, something which was rectified soon after.
Sorry if my response is not what you wished for, but all I can suggest is that you ask. Times may have changed since I did, and all circumstances are different.
The English Passenger