Did you resolve your case?
I've heard that this is a fairly common problem with university's not accepting evidence after a student takes the exam.
It seems to depend on how reasonable your university is with regards to accepting late evidence. I think unfortunately most are very unreasonable. If they have strict guidelines on it, it is very possible that the OIA will defend the university no matter how unreasonable this sounds.
With hindsight, after taking my complaint to the OIA, I wish I would have got a higher education law solicitor to advise me on the merits of my case as early as possible. I know that the OIA advertise on their website that it isn't necessary but I strongly believe they say this for their own benefit not yours. The one I spoke to quoted £300 - £500 to do this, which is a lot of money but worth it, since it is the student’s job to persuade the OIA your complaint is justified. This is very difficult when you are not 100% sure of the merits of your case. I don't think many solicitors have a legal aid contract for judicial review, although I know that at least one exists.
The earlier you get advice the more options are available to the solicitors, as there are many strict deadlines that have to be met. I would even recommend getting advice during the university complaint stage as the solicitors I spoke to said that this gives them more options and also they notice a shift in the university's and OIAs attitude to the complaint.
I believe the main problem with the OIA is that they do not have to make good decisions, or the best possible answer. They simply have to make a decision. I know this sounds unbelievable but I believe it's true. See the judicial review of Thilakawardhana v OIA to see for yourself, if necessary
Also if your SU is well informed get their help as I strongly do not recommend doing this alone.
Please ask if you think there is anything else I can help with