Issue a court summons. Quite a few FT'ers have done this in the past, for various reasons - the last big one was when BA had to charter a few one-class planes and demoted CE pax to Economy with a flat £20 (I think) compensation.

BA ALWAYS settles before the court case is due. However, you will be forced to sign a confidentiality agreement so you will not be allowed to post on here what you have received, and you may even be banned from saying that you have reached a settlement. It is still worth doing, though.