Had my TA change a flight a short notice from a Friday to Saturday evening departure flying LUX-LHR. I was advised by the TA that I needed to get ticket reissued as they had changed the ticket, but could not get confirmation. This was 3 hours before departure on the Friday.
Arrived at the airport, BA's contracted staff, claimed I was a no show for Friday evening, so with not much time I purchased a new ticket for the same flight as I did not want to argue with the contracting staff (I trust the TA).

Roll on two weeks, I contacted BA Customer services and explained the situation. To which they refunded a small amount of the unused ticket price.

What would my options be on this to claim the full amount from BA that I have lost? As by paying a percentage do they admit their error?

Or is it a case of trying to make an insurance claim?