How do you do it?
The scarring along the shores suggested a reservoir, and it looked like the Pennines rather than the Lakes - open rounded hillsides rather than craggy.
So I used an online OS map and quickly panned through likely spots at 1:50,000 scale. As I said, the gully is quite a feature, as is the rounded hilltop with a cairn or trig point on top - this is in line with the gully (another clue). The lack of woods helps eliminate many possibilities, as does the presence of the wall running parallel to the gully. As soon as I saw Balderhead on the map I thought that it was a contender, so I had a quick look at a satellite image - the wall was there where it should be, and I could even spot the trees to the left of the photo! A quick search for images of Balderhead Reservoir provided confirmation.
Edit - the 1:50000 map does show conifers on the western side of the gully - these must have been clear felled some years ago, as they are certainly not present on the satellite imagery - or the photo!
Edit 2 - this image clinched it -
https://binged.it/2IRn1mS