With some trepidation, an external hard drive was plugged into the BT hub. So far no loss of internet, no loss of backup data (also backed up elsewhere), and apparently without crashing large corporate organisations.
The PC internet map could see the drive by name, but not access data.
iPad could not see the drive at all; however, Pages gave an option of WebDAV as a location, but trying many identifiers associated with the BT hub failed to see the disc.
I suspect that WebDAV is irrelevant, the option being displayed without the hard drive in the system; or does this require an App?
So back to sq 1; not quite because with improving proficiency with PC iTunes and iCloud, and also Goo Drive, file transfer is improving. Also, with discipline, the number of files has been reduced.
Now considering buying more iCloud storage and relying on backup hard disc access elsewhere - trusting neighbour / friend.
Oops, auto spell check on 'trusting neighbour' gave 'trysting' ! ... Freudian computers ?)