I too spotted the "tumbling in" employed on the gable brickwork of Don's workshop. This is also seen here occasionally on well built buildings and notably on chimneys and buttresses.
I have thought more about the use of metal sheets and now recall that large 4ft x 2ft galvanised metal sheets were used to apply a cheap weather seal to the exterior of planked wooden siding that was applied the the buck of post mills. This looks dreadful and even worse results in the decay of both the wooden siding and post mill timber frame due to the trapping of moisture within the body of the mill.
