Thanks for all your lovely comments. Purdey is home from the vets after spending 2 days on a drip and I have to say she looks like a playful puppy apart from the weight loss - it's so hard to believe there is anything wrong with her!! She loves the KD diet and bounces as high as the counter top while I'm getting it ready for her!
The only signs (now I'm looking hard for them) is that she has very yucky breath, she does drink a LOT (but she always has), and she is sick a few times a week. She seems a bit uncomfortable at night and struggles to settle down but apart from that, she is chasing birds in the garden, playing tug with Luna, barking at the Postman etc as if there was nothing wrong. I keep clutching at straws and kidding myself that maybe they've misdiagnosed her!! ;-)
I'll PM Bluegirl. If anyone has any suggestions for alternative therapies etc I'd love to hear them. Does anyone know kidney friendly treats I can give her when I'm going out for a few hours - I always used to leave a rawhide chew or kong with pate etc