yeh keeping things in writing does help.
i used to work for the Ombudsman, and 9/10 times they would shit themselves and buckle when I had cases with insurance. I'm seriously suprised with what you're saying. usually the only things we would decline cover for was when we had proof that one panel(for instance) was damaged, and that panel plus 3 other panels with previous damage were fixed under the same claim.
as soon as you get the answer 'no we are not refunding, or reimbursing this amount' the Ombudsman HAVE to take on the case, and it IS within their terms of reference.
FOS then give the insurance company a further 45 days to sort it out internally, and if YOU arent happy with the outcome, then they will further dispute it, given that you have evidence(as you said earlier) they will fight it.