I too worked for one of the Big Four, and I've just put in a claim - I remember distinctly cancelling my Mortgage Repayment Protection once I discovered I didn't actually need it (plenty of savings, death in service benefit, 12 month paid sick-leave, generous redundancy package - them were the days!)
To be honest, I wouldn't bother with a SAR - if you were told you had to have it as a condition of the loan, just say exactly that - imagine how many staff members would have been told that!
I had to say who my employer was at the time, but said I became aware I didn't need it for the reasons stated, and couldn't recall ever having agreed to the credit card PPI. I do have paperwork for the MRP, but all it says is something along the lines of "we discussed MRP and you agreed to go ahead", which is hardly a comprehensive description of the benefits and how they were relevant to my situation.
Also, depending on when you had the PPI, you might be due compensation just for having it (Plevin rules), so even if they reject your initial claim, you might have recourse under that.
You've nothing to lose at all - my Bank even told me which products I had PPI on, so I didn't have to go to much effort at all.
Good luck!