Oh, it wasn't the evacuated school at Denver that I was talking about, it was a preparatory school that Helen was recommending to Harriet for the offspring.
Some at least of the DDD's letters were from the 'Wimsey Papers' which Sayers published in the Spectator, I think, in the first year or so of the war. I found them in a library somewhere, but I do think there were some Paton Walsh additions to them.