My error, I have just re-read the 313 thread again and it is three diagrams they lose with the timetable change, not four. This was confirmed only last Saturday, so I presume this must be independent of any ex SE 377s, which has been known for months won't be happening due to the 701s etc. If 313s lose diagrams, but the same number of units remain operational, then this would seem profligate in todays financial environment. I would have thought it more likely that some 313s remain with GTR as spare parts donors for the rest of the fleet, so GTR may not have got rid of them, but they would be unlikely to ever work again either. The thread says the three diagrams will be 377s, so 387s from GWR to GN, for 387 to SN/GX, for 377 to replace 313, or some alternative permutation, would seem plausible?
I am not entirely sure how GWR cope with three fewer units, but there must have been a plan for 30 4-car units when the tender was issued. In practice, it is only two fewer units because 387174 had been out of service for 90% of the past year during which time GWR effectively only had 32 units to operate the current diagrams.