This is the Longknowe Hill trig point (346m) located in the north west corner of the Northumberland National Park and is the most northern trig point in the Cheviots.
You could park in Kirk Yetholm for this one or if you're travelling from Northumberland, then probably best to park at Hethpool. Follow the permissive path up White Hill before heading west to Betsy's Cairn. Then head north up onto Longknowe Hill.
Just over 4km and 285m of climbing to get to this one and you can follow the route on this map, or find it on OS Maps.
Thanks to Michael Duncan for the photos and information.