Peter Kerslake says "Wolverhampton Stafford Road's 5070, "Sir Daniel Gooch" runs into Bristol Temple Meads on the down "Cornishman" bound eventually for Penzance. The train, starting from Wolverhampton, would have been worked from there by 5070, and it will probably work through to Newton Abbot before being replaced there by an engine running down through Plymouth to Penzance.
I cannot recall that the "Cornishman" engines were changed at Plymouth, and the chances of a Wolverhampton "Castle" working through to Plymouth at this time were pretty slim, as all the "best ones" came off at Newton Abbot, unfortunately for the Plymouth enthusiasts!
A little more research shows that the Stafford Road "Castle" working the Westbound "Cornishman" would be booked to work to Bristol Temple Meads only as it would be expected to return home to Wolverhampton later that same afternoon with the Northbound "Cornishman", its westbound train being taken on from Temple Meads by a fresh locomotive working through to Penznce in all probability".