I know that all too well, but after so many incidents involving this particular train (involving a peak-hour run from the North Downs line), I checked the detailed interaction with other trains between Redhill and East Croydon. The train was timetabled to stand immediately outside EC for four minutes for a platform to become available, and almost always did so. It had no clash with trains at Purley or South Croydon and so a, say, 30 second hold at Redhill would have had no knock-on effects whatsover bewteen Redhill and its booked stand outside EC. I wasn't suggesting the train should have been held no matter what, only when passengers were actually crossing the platform when both trains were standing there. I and others wrote to Southern at the time to point out the farce of passengers being left behind by a train which would then wait outside EC for four minutes but they, of course, had no interest in passenger service; only dogged rule-following which disadvantaged passengers needlessly. It would have been so easy to issue specific instructions for that train to be held in those particular circumstances, but that would have meant a bit of initiative from someone. It wasn't as if a 30 second late departure was anything unusual!
The lack of real concern for passenger needs has been a theme of Redhill's operation for as long as I have used it. The latest example is the farce of the lack of a three-car NB stopping mark on p0 for GW trains, meaning a needlessly long trudge to the stairs, increasing the chance of missing other trains and vice versa (and putting most passengers outside the canopy!). All it would take is an additional 2/3 car stopping mark which would stop GW trains fully under the canopy and with ample clearance from the SB starter (which is where through trains using p0 SB stop anyway) but neither GTR or NR were prepared to take on the huge outlay of a 10cm square sign! The situation persists today and should have been designed into the p0 construction plan. No-one cares about such important (and tiny-cost) details.
Then there is the extremely poor delineation of platform sections (particularly p1) which leads to so much passenger confusion. The DMIs are poorly-positioned and it is only by careful checking that it becomes clear that there are two separate train berths. There are only small DMIs and no additional signage to direct passengers coming up the stairs to the correct berth, who are simply greeted with the DMI for the particular berth they are at - nothing directing them further along the platform to the northern-end berth. I used to design and implement station planning and signing schemes for LU, and the one at Redhill is an utter shambles.