I am overjoyed to announce that Ms Jade is doing fine - better than fine, she is curled up on her favourite (forbidden lol) angora rug near the heater, snoozing and purring.
Turns out it wasn't a kidney stone at all, it was just a bladder infection! They put her under at the vet surgery so they could get a urine sample (poor bubba was holding it since last night 'cause it hurt so much!), and had called me to advise me of this and say they wanted to perform the surgery at the same time, to avoid a double anasthetic. I gave the go-ahead... and then got the call a couple of hours later telling me that they has evacuated her bladder and couldn't feel any stone there so didn't even need to operate. Yay!
She has been given painkillers and a long dose of antibiotics, and special cat food to stop the bladder infection happening again. V and I are so relieved and so thrilled to have our girl home again.
Much love to you all
EDIT: I just have to add, I have realised that I think Jade is fairly STONED at the moment haha :D The anasthetic is obviously still wearing off! I gave her a cuddle a moment ago and she was like, Duuuuude why'd you move me? Lol