Thanks for the clarity.
It's something you should know given your qualification BUT.....
You were hired with the knowledge that you had no practical application.
Presumably, this is why you were provided with someone to oversee your work.
If you didn't need supervision, it wouldn't have been put in place.
That goes in your favour.
Secondly, it wasn't deliberate or malicious.
Thirdly, you weren't aware and didn't try cover it up.
You have also carried out additional training since to avoid a repeat.
I think the implications and negative financial implications will play a roll here. If you had to quantify the error, is it a once off say £X amount. Or is it ongoing, annual cost due to the advice? Can it be rectified & is there a cost to that too? All will probably be factored into the decision.
What has your work and feedback been like since? If very positive, this may be a mitigant.
If they were to audit say 20 other files, would you be confident they are perfect. Could you suggest that of it's needed?
Also, if worst comes to worst and it looks like you are being fired? You need to ask for departure by mutual consent.
It's very difficult for any of us to know the outcome and the culture will play a part. If they are looking for a scapegoat, you are it unfortunately. Hopefully, there's someone internally who will fight to keep you.
Is your manager saying much? Are they the person who signed off on it?