Regarding only needing 4 67s for Mail trains.....
Right up to the end of Mail trains there were 4 needed at St Blazey, 2 at Bristol, 2 at Swansea, 2 at Norwich, 1 at Tyne Yard, 1 at Motherwell, 4 at Willesden. There were then 2 needed for the Sleepers and 1 for the Walsall - Aberdeen fast...