I have good news
Biscuit has seen both our physiotherapist and vet and she is still with us!
Sadly it seems that her disability is most likely due to neglect rather than a permanent problem. We had no idea from what we had told that the disability would be caused by this. We thought she was disabled from a health issue and the owner couldn't cope with the additional care needed not that she had been left to suffer so badly.
Her body is weak, she has no muscle at all. But in just the few days she has been with us we have seen improvement. Her front legs are now able to get her into a sitting position
She is also managing to keep herself clean underneath.