I'm an infrequent BA traveller (usually fly VS to the America's), and am having something of a tussle with them

I paid £60 to reserve two "twin" seats on BA296 (ORD>LHR), as my partner and I prefer these seats to 3-across where you can get "trapped in" by the window. When I reserved the seats, the seat map for the plane was a 777-200.

When we boarded the plane, we found that it had been swapped for a new and longer 777-300, and whilst numerically we had the same seats (39A, 39B) they were no longer the twin seats I had paid a premium to reserve (i.e. there was an occupied 39C next to 39B).

I raised this with the cabin crew and they informed me there had been a last minute change of aircraft and unfortuantely there were no twin seats to move us to. She said she was sure we would get a refund though.

So I applied online for a seating reservation fee refund, but BA are not playing ball. They're taking the view that we paid for 39A, 39B and got these seats so there is no issue. I disagree on the basis that the seats I reserved were more expensive to reserve than other seats on the plane because they were twin, and then we winded up in non-twin seats.

Am I being reasonable seeking a refund here? Does anyone have any tips on getting this to a successful resolution? I have no status (other than blue) with BA, and given their handling of this I think it's unlikely I ever will!