I was involved in clearing out the basement of no.7. Most of the music and paperwork is now in the Cathedral Office basement, waiting to be sorted, but I don't think there were many manuscripts - with the exception of a briefcase of music by H.A. Bennett which was left to the Cathedral. We do have what seems to be a complete record of all the Rochester Cathedral Chant Books, which includes one by Luard-Selby, Hylton Stewart, Bennett and of course Ashfield.
I was at Mary Ashfield's house last year, helping to put together a pile of the music previously published by Oecumuse. Whilst there I did come across said orchestral parts/score - not in RJA's hand. So they do exist!