In order to do this cache you will have to complete the series of Adventure lab caches called Slitting Mill. A Hamlet with Heritage. The information needed to find the coodinates will be found in the journal entries after completing each stage.
The cache is placed close to where the furnace for the slitting mill was placed.
The stages can be done in any order, but I would suggest that if you want to do the series as a circular with the bonus at the end, that you do the stages in the following order.
St John the Baptist Church
Stonehouse
Pumping Station
Horns Pool
Waterfall
The cache can be found at
N52 AB.CDE W001 FG.HJK