Mummybunny, it definitely sounds like Marek's to me.
It is probably too late to isolate the Light Sussex hen now. She may not get it anyway as it seems mainly to affect Marans. Four of my six Maran chicks that I bought from the market had the disease and died. Two of the chicks and the other chickens that I already had were all right though.
After that episode, I steered clear of Merans and, as I already had a Welsummers cross Rhode Island Red cockerel, I bred my own chicks even though they were cross breeds and haven't had to see any more of my chickens suffering with Marek's.
I hope the remaining hens that you have do not go down with the disease.