Don't know, but I'd hope that BA would:
a) re-assign that traveller's seat to a non-bassinet position,
b) then refund their 50 quid
c) and not charge me (or the infant!) 50quid for the seat.

Will be interesting to see how this pans out in practice though. I can't imagine they'd say "Sorry sir / madam, someone has paid for more legroom so you can't put your baby in the cot on your 9 hour night flight", but you never know. Race to the bottom and all that