Thank you @YesItsMeIDontCare . I've pretty much decided (with the help of family, friends, and The Litter Tray) that what you do is how I'm going to play it. Leroy is a magnificent cat (we're overdue some pics I think!) and I doubt I could care for Tobias any better than you care for him. Only real difference is Tobias has become a fully indoor cat long before this diagnosis so I don't have to work out how to get a spitting, clawing & biting demon into a harness!
Luckily I had him insured from day one of him coming home so this isn't a pre-existing condition. My premiums may go through the roof next year though.
The only exception is that I think I will have the GI Tract scanned. The only real symptom Tobias has had is the poo issues which is unusual & hasn't responded to all the usual methods like grain free, gastro food, pro-biotics etc, and was the reason he ended up being tested.
However I will get that done locally, not going to stress him even more by going to Potters Bar. He tends to respond pretty well to Cathy (usual vet) and is used to her so she will be with him when the Ultrasound specialist sees him.
Girlcat gets tested for it on Wednesday. No symptoms, but if she does have it that will mean she goes on to the twice yearly MoT regime now rather than later in life.