Thanks everyone, took him back to the vets today as although he is nibbling at food he is still very down/lethargic.
The vet gave him another steroid jab and said that given that he is young, fit and otherwise healthy he suspects it's a long lasting form of gastritis and that if he is no better by weds then to bring him back in for tests. He wasn't dehydrated this time which is good and he hasn't been sick since I bought him home on Saturday.
Went to the shop and bought him cat milk, tuna (in brine as had already been before you posted Yvonne), beef slices and gourmet gold. He ignored the stewing steak that he ate yesterday but has had a tiny nibble of normal cat food.