Why does 387s to Cardiff come up all the time? There aren't any self contained diagrams that could work all day, other than the partial diagram already allowed for starting with 0712 from Paddington to Cardiff. By the time the second train to Cardiff is leaving London, plenty of IETs have arrived from the West. The last train to Cardiff is the 1518 from Paddington, after which all services run to Swansea. GWR can't reduce the 80x requirement to any great extent by running 387s.Perhaps I'm being thick but aren't GWR also short of / going to be short of units due to Hitachi units going to Eastleigh (or elsewhere) for cracking repairs and the Class 387s to Cardiff would cover for this?
GWR will have taken the need for remedial work on Hitachi units into account in their plans for using them in the west, without the need for more 387s to Cardiff.

