Thanks for the replies. To take a few points in order:

Prospero - I find it interesting in the notes on BA's website that they detail a seperate policy for exit row seats (which also incur a premium to reserve), but not window pairs. Is charging extra for window pairs relatively new? The FA definitely didn't know they charged extra when I first mentioned it. My argument is that BA reserved both 39A and 39B seats at the twin rate but neither seat had the attributes of a twin seat when we boarded. Therefore I do not consider the seats we were allocated as "suitable".

BA-Flyer - That is the form that I used. I think the form is badly designed because in my case the seats I reserved and received were numerically the same, and none of the four options listed really explain what my problem is. They need a "Layout Change" option, and maybe a free text box for additional information.

dastonm - Any ideas the best way to convince BA on this? I'm thinking calling up on Tuesday may be the best option rather than constant emails.

Essex.