Having just done a similar trip last week I was on the 1240 LHR - MAN flight with a booked arrival time on 1345 for a train at 1406. Although we landed early we were delayed taxiing to the terminal due to another aircraft blocking our stand, and then the guidance system crashed so we had a further delay. As it happened, doors opened at 1345 so a bang on time arrival. I was hampered a little by people faffing around and not disembarking promptly etc. but a brisk walk saw me on my train by 1400, so six minutes to spare. It was a very brisk walk but it would have been quicker had all the moving walkways on the Skylink been working.

In terms of easements for late arrivals, I know that Transpennine Express (TPE) has a three-hour easement for advance purchase tickets starting at Manchester Airport. Hence if your flight is late you may travel on a subsequent train departing up to three hours after the original service. You need to have your ticket endorsed by the station staff. You won't have a reserved seat.

Hope that helps!