I thought there might be a location clue as on the 1750 map I can see "Tearth Water" beside Garrelfoot. Googling Tearth water throws up all sorts of irrelevant results. I can't see it marked on any other maps.
A couple of places on the 1750 map still exist today. I think being flexible regarding the location might be the best approach as I think the mapmaker used a bit of artistic licence.
Something that may or may not be relevant is that on maps around the mid 1800s is a Garvald Burn which you can follow on maps NE from Fernyhaugh. Maybe Garvald sounds a bit like Garwell? Maybe a coincidence?
On the 1750 map it looks like Garrelfoot is just NW of Ingraston, but I suspect it might be further away than it appears.