Not my area at all but big black cats are actually spotted fairly regularly over here in Lancashire too, we call them "phantom cats" or ABC's (alien big cats, nothing to do with actual aliens just means out of place). Nobody ever believes it until they see one.
The thing that gets me is how consistent the descriptions always are. Sleek, jet black, much bigger than any domestic cat, moves completely silently. Your description sounds exactly like what people here report near the Forest of Bowland.
Have you managed to get any photos or even just footage on your phone? Even blurry stuff is worth posting up. Also check your property for prints, they leave massive ones apparently and casts have been done before with decent results.
Would love to know if anyone else near you has reported the same thing recently, these sightings tend to cluster together which suggests actual animals moving through an area rather than just one-off misidentifications.