Thanks Damascus - sorry for the late reply. I was realising as I was writing the post how impossible it would be for anyone else to even guess as to what it is that she's up to - but I'm just a bit desperate to know for sure what it is that we're dealing with...
It's so hard to know if it's drugs or mental illness - or as you say, a combination of both... But the thought that it is perhaps just mental illness keeps Mum & Dad hanging on feeling guilty that they're abandoning her when she's unwell.
I suppose the end result is still the same, if she isn't willing or interested in helping herself.
Thanks again for your response