I just looked up the S.28 in my Putnam "Shorts", and this is what it has to say.....
"In preparation for the next generation of long-range airliners, the Air Ministry issed two specifications, 14/38 and 15/38, for the Empire and European routes respectively. Some months before, the aircraft industry had put forward various proposals for a four-engined monoplane successor to the Ensign, which had become out-dated almost before entering service because of its delayed delivery; among these had been one from Short Brothers with four Armstrong Siddeley Terrier three-row-radial engines, but this project (design index No. S.28) had lagged because the engines never emerged from the design stage and was dropped altogether when the Stirling prototypes were ordered."
I have looked at my database and seen no requirement for an "airliner" a few months prior to 14/38 and no other info on S.28.