Sorry for dredging this thread up, but I thought about it the other day and wondered how long ago it was that I posted it. Turns out, it was exactly a year ago.
The good news is that Roxy is still very much alive and she's much healthier and happier than she was a year ago. The only 'treatment' she's had has been milk thistle, and I'm convinced it's helped her. She still has her off times and goes off her food, but they only last a few days and then she gets her appetite back and perks up again. I've given up trying to feed her the healthier dog foods and she now has whatever she wants - tins of Butchers are what she'll eat most reliably. She also has chicken and cous-cous (cous-cous is just about the only non-meat item that she'll eat, other than KitKats, and I obviously don't want her to eat many of those; although I'm wondering if she was weaned on KitKats as she always perks up when she sees one, and she is from York (home of the KitKat) ).
I might take her back to the vet for a blood test soon, to see how the results compare to last year.
Hoping she's still around this time next year!