Thanks, that was the total return fare, so presumably (1855+550)*0.5*.75? Or would I need to get a further per leg breakdown?

I was also curious as to whether any taxes would be cheaper with PE, and if so is it possible/acceptable to claim for that difference too?

I'm actually claiming on behalf of my other half and her parents. I'm not sure of the layout, I will speak to her today, and get the boarding passes. The downgrade had to be accepted for both the IB and BA bookings, and certainly in BA MMB (she is booked separately) the flight was classified as PE. Email communications mention compensation would be due.

If it was 1-2-1, would it be unethical to claim, not sure, the whole booking has been a bit of a nightmare with little help from BA or IB, to the extent that IB had to issue her a new ticket at checkin, since the existing reservation was so broken.