I'm currently trying to read your post, I have no clue on how to deal with this if it is mareks, when I got my chickens I had one blind one, and about a month ago I had my 1/2 battery hen have a prolapse and infections with ulcers in the vent, the vet said if it didn't heal then it could be tumors, and I would have been more than happy to pay for surgery as crazy as it seems. If it could have been tumors that's mareks all over and now this one aswell
Quite a few of us deal with Marek's in our flocks and still enjoy chickens. If it's Marek's, the very best thing you can do is to find out for sure, and educate yourself on the disease so you know how to deal with it.
It might not be Marek's, in which case you'd deal with the problem differently for the most part, so finding out for sure would be great if you can. There are things that "look" like Marek's disease, and so you must consider that there are several possibilities.
I know this is frustrating and you are just looking for an answer!

If there anywhere in England that will do testing for you? Here in the states they can test via necropsy of a dead bird, or a blood sample from a live bird. I don't know what resources you have over there.