This is an unusual situation I think. The Consumer Action Group website has a story about FOS poor service received by a complainant (see link below). The complainant wrote to Caroline Wayman who seems to have instructed that an FOS Team Leader should take another look at the case. The team leader found there were mistakes and offered a correction and apology.
I don’t suppose writing to Ms Wayman gets this kind of result very often, but its obviously worth thinking about !
Decision overturned Link to full story on the CAG website