I got a call from the vet today (well the wife actually spoke to the vet) with the results of the post mortem on Nobby. The good news is that she didn't die from anything contagious or preventable.

The cause of death was septicaemia. The septicaemia came about from a large number of septic cysts which filled most of her lower abdominal compartment. The cysts originated from her ovaries or the ovarian ducts, and are likely to have been building for several years and possibly for most of her adult life. There were a lot of follicles that had failed to develop into eggs and had become septic. There was no external sign despite the extent of the cysts, they couldn't be felt, so the first visible sign was the petechiae in her mouth when the septicaemia set in.
The rest of her organs were all normal. The vet said that had they investigated sooner there would have been nothing that could have been done to save her, certainly their recommendation would have been euthanasia had they performed an endoscopy and found the issue.

Nobby was another example of a snake that hid a long term illness for a long time whilst appearing to be healthy.