Yes, actually I am one of the moderators in a yahoo canine kidney failure group......
Can you get a copy of her blood chemistry report and urinalysis and post all of her values? With kidney failure, you will see an increase in BUN (blood urea nitrogen), increase in creatinine and increase in phosphorus. You will also see a lowered urine specific gravity (indicates inability to concentrate urine) and a lowered hematocrit (also called packed cell volume) as kidney failure causes anemia. Depending on what these numbers are, kidney failure is staged according to the severity. Kidney failure can be chronic or acute. Acute failure can be caused by infection, a kidney injury (such as ingesting a toxin or a drug reaction or leptospirosis or a tick borne disease), etc. Acute failure can be reversed in about half of the cases, but about one-fourth (if I correctly remember the stats I learned at AVMA convention) of acute will lead to chronic failure. Chronic failure is related more to aging or a result of poor perfusion from heart failure or similar.
Increased BUN reflects dehydration, and an increase can be caused by many things. Long term diuretic therapy almost always causes BUN to rise above normal values. I've seen this in my own dogs on heart meds. I personally would be very reluctant to use furosemide (US spelling) on a long term basis for SM, but then again I've not had a dog with SM and I have had dogs with kidney failure. And I know what long term furosemide use does to kidneys. Pred can be hard on kidneys also I think (I'll have to check Plumbs when I get home.)
Creatinine is a much more accurate indicator of true kidney functioning. Urine specific gravity is a measurement that tells you how well the dog is concentrating urine. A low USG is the first sign of kidney problems, and it shows up long before blood chemistry values are abnormal. (This is why I always run urinalysis when I run blood chemistry, and you need a first morning urine sample to accurately measure USG.) When BUN, creatinine and phosphorus are above normal, generally one only has about 25-30% kidney function remaining. But dogs with chronic kidney failure can be maintained for a long time with a reasonably good quality of life with good treatment and a little luck.
It would be odd to have elevated phosphorus from kidney failure without having elevated BUN and creatinine. Elevated phosphorus happens when a dog's kidneys aren't working well enough to clear the waste/toxins from the blood. Elevated phosphorus causes a dog to feel nauseous and unwell and not have an appetite. Foods that are high in protein are also high in phosphorus, but the newer thinking is not to feed low protein foods per se but rather to try to feed a diet that is higher in good quality protein (such as egg whites) but lower in phosphorus. (Also commercial kidney food is very high in fat which can often trigger pancreatitis, which is why I prefer a home cooked diet for dogs in kidney failure.)
There are several ways to treat kidney failure - low phosphorus diet, some meds that help the symptoms, giving IV fluids at the vet's and/or subq fluids at home (done commonly in the US but for some reason UK vets often resist letting their clients give subq fluids at home) and using a phosphorus binder.
I believe the most effective phosphorus binder is aluminum hydroxide. It must be given with food or immediately after the dog eats in order to work. The liquid (Alternagel) tastes bad. I have always used alum hydroxide powder in capsules and coated the capsules with raw honey. (Don't use cheese or deli meat, etc. - very high in phosphorus.) The binder takes phosphorus out through the gut so the kidneys don't have to work as hard. I always gave it immediately after my dog ate rather than mixing it in food (tastes bad) thus risking rejection of the meal. Other products that are supposed to bind phosphorus are things like Epakitan, Azodyl, etc. but they are not as effective (in the experience of many members of my kidney group).
I have had several Cavaliers with kidney failure, so I've been down that road. My beloved boy (Capers) had end stage heart failure and concurrent kidney failure, but with a wonderful cardiologist and good treatment, his symptoms were well under control and he ended up being PTS at 16 1/2 for other reasons.
Does she have any symptoms now? How is her appetite? If you can post her exact numbers, that will tell me more, although I have a hard time with UK lab values while US lab value meanings are pretty well burned into my brain as far as staging of disease. I do have some expert sources that I can consult. There are some great websites - Mary Strauss' Dogaware site and Tanya's UK feline kidney failure site are both excellent.
Sorry this is so long - it's a big topic. Probably way more than you wanted to read, but this subject is one to which I've devoted years of study.
Pat