I am repeating the professional legal advice that I received, which similarly related to a missed onward connection after a flight delay that was the airline's fault. I've received three payouts from airlines in the last 9 months for missed connections under Article 19 of the Montreal Convention. BA paid up the fastest. So the OP should definitely look into this, irrespective of what you think.
ok, i conscious of labouring the point - but this is very significant if it is correct. to be clear:
- you had two completely separate bookings for flights?
- the flight on the first booking with BA arrived late or was cancelled causing you to miss your second flight booking (any detail on what happened exactly, plane went tech? ATC restrictions? how long was there between the bookings as scheduled?)
- you then submitted a claim to BA for the total cost of the missed second booking based on Art. 19 MC99? was this just a claim for the cost of the second booking or were there other associated costs included such as hotels etc.?
- BA paid the claimed total cost?
- did BA pay just on the basis of written requests or did you have to commence legal action, and if so was that CEDR, MCOL, etc? was there an adjudication/judgement (details may be useful to see if so) or did BA settle your claim prior to adjudication/judgement?