We have had a similar discussion about this in the past:
http://www.firstgreatwestern.info/coffeeshop/index.php?topic=1400.0However in order to reply to this train specifically please note the following:
First of all, the train is formed off 2116(?) arrival and splits to form 2127 & 2142 departures.
Plat 4 & 5 - only used for local services on Sundays when as you may know, the timetable is based on Mains or Reliefs not being available. So during the week at this time with Heathrow Express still at full intensity, delays will be caused crossing a class 2 across the throat. Also, the platform may be empty when you arrive, but that does not mean that a
HST▸ wasn't there when the 2116 arrived, or is booked to be in either platform between 2116 & 2142.
Platform 10 - 2115 Greenford is due to depart this platform, so the 2116 arrival CANNOT be planned to arrive into it 1 min later.
Platform 11 - the 2320 Oxford is due to stable here - so the 2116 (can't remember if its 5 or 6) arrival cannot be planned to arrive on top of it, as it will mean that access to plat 12, where the connect is due to arrive at 2124 would be blocked.
Platform 12. Heathrow Connect is booked to occupy the platform between 2124 & 2133, so 2127 & 2142 departures cannot occupy it.
As to the trains on 14. the stock for the 2210 is due to go to the depot at about 2000, and return for the 2210 (booked off 14). So if there are driver issues, it is often stabled on the blocks of 14 until 2210, with other Plat 14 workings going in and out on top.
As for late boarding, procedure dictates that trains are not boarded until the split is completed, or at the least the rear unit locked up.