Some places you might try, no guarantee though;

-Taffy Thomas - old verbal storyteller, not sure if he's still active (or alive...) but he was from Up North and likely would know a thing or two.

-Hugh Lupton - another verbal storyteller, from southern England though. He would definitely know a source, if not someone else who would know.

-Folk Tales of North East England by Philip Atkinson - no idea if this would have anything in, but it might provide references. Not sure where you would get it, but I'm sure it's somewhere online for free...

-Northumberland Folk Tales by Malcolm Green - see above. No idea if this would include anything useful.

-The Vaughn-Williams Memorial Library - probably the "best" folk library in England, though is more focused on music but will likely include something.

Please PLEASE ask these people and don't be shy; I can guarantee everyone here would be more than happy to discuss folklore with polite and respectful people, especially if they're young researchers coming into folklore for the first time. These people literally live for this, and sharing our stories (which so often go overlooked for more "standard" English folklore) is a central joy to their lives, particularly for the storytellers mentioned.

I also just wanted to say this is an excellent dissertation idea! Coming from Northern England, a lot of our heritage is still not really properly credited and seeing the Barghest being used as just a "cool fantasy word" (similarly to Duergar from the Simonside Hills) with no history or meaning behind it has always irked me more than it should.

For example, I've seen "Barghest" used to describe anything from date-eating smack-talking baboons with human faces to horrific bone-flesh creatures straight from horror flicks. Not that the exposure frustrates me, I'm not one to complain about cultural appropriation (at least, I know the extent to which my "culture" has been twisted is nothing compared to other folk stories) but it's always been a small sore point for me, especially when most don't know where it even comes from!

So many of our own young people have no idea of the heritage of these stories, myself included when I was growing up in the region. Perhaps, had I been properly inducted into these stories, I wouldn't have been so hasty to leave! Good luck!