Riboflavin deficiency is best treated when symptoms first start, and you should see improvement within a week or two. I hope that is the problem. Here are some articles about Mareks to read, and the second article includes diseases that can look like Mareks:
Sadly, Dorothy didn't make it I'm totally gutted, she was such a sweet little girl but I couldn't let her continue to suffer in the hope that in a few weeks there may be some improvement, she couldn't even sit up straight in the end and it all happen soooo quickly, literally in the course of a couple of hours, she suddenly went downhill.
Having done loads of reading on Mareks, I think that there is a real likelihood that this was the problem. I've disinfected the coop as best I can, I did this last weekend when I first separated her from the others, I realise that as my girls (and Ken) are free range then the virus is likely to be everywhere, but is there anything else that I can do now to help those remaining? I would like to replace Dorothy at some point, but obviously not going to do it for a while under the circumstances, how long is safe to wait? I'm amazed at how many posts on here are similar to mine, this horrible virus is obviously very common and perhaps I've just been lucky up until now that I've not had a problem.
Your state vet can do a necropsy on a chicken to look for Mareks if the body has been refrigerated (not frozen.) In a few states, they are free. Hopefully she did not have Mareks, but if you suspect it in any other chickens, make sure that any new chickens in your future be vaccinated for Mareks, and kept away from any contaminated area for 2-3 weeks until they are fully immune. I'm sorry for your loss. Here is a link or two about necropsies: