Peter kerslake says - I think this could well be Hemerdon Bank just east of Plymouth as the leading Royal Mail coach indicates that this is the 12 noon from Penzance , the " North Mail " , worked forward from Plymouth North Road at 3. 45 p.m. The train has every look as being this working as shown in many of my pictures of same.

I think that I am correct , and this being the case the " Castle " will work through to Bristol Temple Meads , coming off this " West to North " working there , that engine and its footplate crew working onwards from Bristol to Paddington on a later duty that same evening.
The Travelling Post Office and leading coach will also be detached at Temple Meads , returning to Plymouth the following morning but 6815 is helping over the South Devon banks to Newton Abbot only on this invaraibly heavily loaded daily working