It occurred to me that perhaps the oatmeal required had to be a mixture of coarse & fine (I do not know the state of the art of grinding in Staffordshire in the early 19th century). So I tried mixing in some coarse oatmeal with the oat flour, and I used a 4:1 ratio of milk to the mixture. The result was somewhat chewier, but there was no "puther of dry oatmeal". As a next experiment, I will try removing the saucepan from the heat immediately after strewing the flour.