It was my original post. Posting from another computer so no mudcat name.   Anyway, many thanks for the replies just working my way through.
We've ruled out bird flu, we kept hens for the best part of 20 years so know a little bit about them.
The hens is around 5/6 years old.
She's still alive this morning and acting completely normal, eating, drinking, clucking etc. Even has a go at the dog as its walks past. Just cant stand up.
Will check out Marek's Disease (neurolympdonictosis).
Our friend is a vet she took a look last night, but had to admit she's never come across anything like this before, usually because if a hen is not performing it meets its maker long before it meets the vet. She's going to do some research today and get back to me.

Many thanks for all the replies I'll let you know how she goes on.