Update - this morning he has at least eaten something - first since Saturday.
He spent all yesterday asleep after the visit to the vets. He did sit outside with me in the garden while I planted some plants but other than that he didn't move. He didn't eat and would not go outside for toilets. We couldn't even get him upstairs or outside so he slept at the bottom of the stairs moving no more than five feet all night.
This morning I could only move him by putting his lead on and gently leading him outside. He has at least managed a poo and a wee at last, but he is not at all well. He is all scrunched up with his back legs looking most peculiar and he is very unhappy. He struggles to get comfortable at all.
Vets at 11.10 this morning so we'll see what happens next.