Never been to that specific mill but I grew up near Carlisle and spent a fair amount of time poking around old industrial sites as a teenager, and shadow figures in abandoned mills are genuinely one of the more commonly reported phenomena in that category of location. Something about the combination of low light, dust particles, crumbling architecture and your own brain running in high-alert mode creates conditions where you're almost primed to see movement that isn't there.
That said I'm not just writing it off. What exactly did you see? Was it peripheral or did you get a direct look at it, because those are very different experiences neurologically speaking. Peripheral shadow sightings have a pretty mundane explanation involving rod cells in your retina but a full straight-on sighting is harder to dismiss. Did it move with purpose or was it more like it just appeared and vanished. Did you have anyone with you who also saw it?
Would be good to get more detail before people pile on with the "definitely haunted" or "definitely not haunted" takes.