I think that one possible answer to Craig's problem of the disparity between the seat tube wall thicknesses of his Bate of London and his Jack Taylor, is that the Bates is built from Imperial dimension Reynolds tubing ie the seat-tube will have an OD of 28.6mm while Norman used Metric dimensioned tubes which would have a 28.0mm OD seat tube. Taylors were known to use metric tubes on occasion especially in their tandems.
The Imperial sized 531 PG tubing, when correctly reamed, should take a 26.4mm pillar, while the Metric one will take a 26.2mm.
That's the best shot I can give the problem at the moment.
Noris Lockley
Settle UK