Many thanks for your comprehensive answer. The one query I had was that in both gastric motility and pyloric stenosis it looks as though one of the major symptoms is regurgitation. In case of my dog he's not regurgitated at all and he doesn't appear to be experiencing any abdominal discomfort as the vet has thoroughly palpated that area on each visit with no visible signs of distress from him. He also has canine massage weekly and he's a completely relaxed state when she's working his tummy. We haven't found any particular area of pain other than the arthritic areas in back and legs we already knew about.
As a follow-up, this morning he shunned his breakfast of lamb heart - but oddly went off into the garden and ate some pieces of lamb heart that I'd tried to feed him last night and had thrown out into the garden for the crows. So it's as though, if hunger absolutely forces him to, he'll find something to eat but something 'scavenged' rather than presented in a food bowl or by hand.