This trip is a fairly simple DUS - LHR (return) - albeit booked as a BA holiday as I need a car. MMB shows I have seat 10A and 13C, so my cello will have to be seated seperately. It seems I'll also have to leave my cello in London as the issue only exists on the outbound.
Interestingly, checkmytrip shows my seating as 10A, unconfirmed