Update on Jakey - he is doing OK, over the weekend the vomiting and excess salivation that had been worrying me in a "this is urgent" way mostly cleared up and hasn't returned as of now, so I don't know what all that was about but whatever it was doesn't seem to be bothering him now! So that is excellent news.
That being the case, I've left his ultrasound appointment booked for 23rd May, rather than pester for an urgent appointment - of course I can always book him in more urgently if there is any decline or he develops more worrying symptoms, but for the moment he is doing well - eating well, litter tray habits and products all normal, drinking a normal amount, he's on early renal dry food already as suggested by the vet and we're getting some of the wet for him to try next week as he does love the wet part of his diet.
Still concerned about the weight loss and slightly high numbers on a couple of indicators on the blood test of course.
So yeah, ultrasound on 23rd May unless anything goes (more) wrong in the meantime.