I know I mentioned she went for a weigh-in at the vet, and our incredible vet said "have we tried putting her back on her old food?"
She used to eat Core (after several changes of food) and she suffered from skin problems (this has been a long term condition, we changed foods several times).
We put her on venison food and her skin cleared up immediately. Apparently she also stopped eating at some point.
She's been back on Core since last Thursday or so and is chomping away. Hooray! Plus we have not seen any evidence of skin problems, but it is still early days.