Well, that recipe's results would not qulaify for a low fat cereal, would it?
Does your AGA use peat or coal? How often do you have to fill it with fuel if you are just cooking? I guess it takes more fuel in the winter when you want more heat. Don't you have a slow oven and a hotter oven? Seems like 275F would be close to the slow oven's temp, maybe. It is basically a drying process.
Quaker, the maker of puffed cereals, used to have neat TV commercials in the 1960s set to the later part of the 1812 overture, after the longest cadenza in musical literature, with the cannons firing over the marching music and the puffed cereal flying all over. "This is the cereal that's shot from guns, the Quaker guns that make it good to eat..." I don't remember the rest.