I asked at T5 on my outbound flight to see if I could change the final segment of my return in 4 days time. My reason was that the layover was only 60 mins on the return and I would prefer more time to ensure my bag got home with me. I was on a flexible Y ticket. BA declined to make the change saying that the MCT was 45 minutes, so I therefore had enough time and they could not make the change as the system would not allow it. They suggested I wait until the day in question and ask again if my inbound was late at LHR.
So flexible tickets don't appear very flexible either. In the end my inbound to T5 was early and I actually had 80 mins. I did make my flight ok, but my bag did not make it with me as suspected.
Route was ABZ-IAH-ABZ