The day before yesterday I passed the outlines of our problem to the passenger complaints section of the CAA.
The response came "We are currently taking about two months to reply to enquiries"
That explains why BA were unimpressed by my previously saying that I would refer the problem to the CAA!!
Also yesterday we told BA that the offered £250 discount voucher was worthless to us as other than already booked and paid-for flights, Much Beloved has no further travel plans involving BA in the next 12 months (the life of the voucher). We suggested in a spirit of "reasonableness" that if BA offered £250 cash instead, we would forget the denied boarding compensation and the additional costs (45 pence/mile for 270 miles) of collecting MB from Glasgow.
On their performance to date I'm not expecting a positive response from BA so we will then go the breach of contract legal redress route as suggested by Hipennine. However the proffered "reasonableness" won't hurt in court.