Spoke too soon it seems!
Having directed Serviio to the Primary blue "Pictures" folder, it can access relevant sub folders, but then it seems to have a problem with some of the "Sub-sub-folders".
Some of the ones it can see, it tells me don't have any content (not true!) others it just refuses to see.
One of those sub-sub folders it can't see is called "holidays" and contains more folders within, named by Place and Date.
In order to try and access these, I made a new Library Path direct to the folder "holidays"...it's now listed in the folder view on my TV, but again when selected, says there is no content....I don't know how to persuade Serviio to "put on it's reading glasses!"
Today, Serviio appears to be sulking, it seems to be working on my PC, but the TV cannot access it.