How about: remove the composite std/1st/former buffet coach from one of the existing 7 coach HST sets - already PRM compliant. No significant reduction in std class seats (only 8 std seats lost, and arguably removes over-provision of 1st class)
Make 4 spare TS coaches PRM compliant (? could be done by end of 2020, maybe) and add the composite.
Then you've got an extra 5 coach HST set, voyager timings, with more std class than a 5 coach voyager (I think) to replace one of the 'moderately' overcrowded 5 coach voyagers, and that then frees up a 5 coach voyager to double up on the most overcrowded 5 coach voyager diagram/s.
It's not much, but it's got the advantages that
a) It *should* be relatively quick and simple compared to the pie-in-the-sky of new/other cascaded rolling stock which never seems to happen on XC. With the composite already done, it's just 4 coaches to be PRMed!
b) It doesn't require pathing changes, which seem to be very difficult for XC for obvious reasons
c) No staff training required, no obvious maintenance implications.
It is starting to sound as if something like this might be planned.
Anyhow, maybe I'm missing something obvious which would make this impractical.
I can see one disadvantage, which is that you've now got 1x5, 1x6 and 4x7 coach HST sets and only one set that can really keep to voyager timings, but you might be able to deal with this by re-forming sets and rotating coaches when they are due for maintenance (e.g. reduce the 6 coach set to 5 coaches when the 5 coach set needs maintenance, so it can keep voyager timings). I've no idea how practical this would be but it sounds possible. Maybe another combination would work better.
