Lots of good thoughts for Marmite.
I lost my 12 year old Birman cat in April,. he was literally skin & bone, a month before he had been diagnosed with a tumour, the vet could feel it, so pretty sure what it was. He probably wouldn't have survived further investigation, although he had blood tests which further indicated the cause. However, he didn't really go off his food until a couple of days before he died. I'm assuning the vet has tested for hypothyrodisism?