fishbangwollop, well done, I believe that it is indeed Talla res.
Well I'm going to be that irritating pedant and suggest it's not Talla Reservoir. I reckon the picture is taken from about 2nm west of Talla VOR, looking east. The highest hill on the skyline centre right is The Cheviot, and the body of water in the middle distance centre right is I think St Mary's Loch. Megget Reservoir is closer but I think hidden in the valley, and Talla Reservoir is almost under the aircraft, off the bottom right corner of the photo.
Been there a few times, usually scratching under a 3000ish cloudbase!
NS