I got it working again in the end, not sure how.
A mate has given me access to his libraries that have thousands of files on, so I plan to give that a go. Will be the first time I’ve used Plex in this way as opposed to casting from a laptop to a tv in the same room
Just had fibre installed locally so speed shouldn’t be an issue from next week when I’m switched to that, 900 meg I think it is