TSM▸ ...

this was shortly followed by 1A96 1954 Exeter St Davids - London Paddington starting from Taunton, instead of Exeter.
1C88 forms 1A96, so assuming another set was used starting at Taunton

Ah yes it does, I forgot about that!
Not sure what they used to form it, I'd imagine it was most proberly the
HST▸ from 1C22, which normally runs empty back to Laira and was proberly stuck behind 1C88, so was reversed somewhere to go back and form it, as it did depart Taunton about half hour late. This leaving the set from 1C88 to go to Laira.