The lane that you would have taken from Charlton, or across the fields from Offley is Windmill Lane, so-called because Charlton once had three mills (or perhaps four if you count the Temple Mill at Wellhead). There was a watermill in the centre of the village (the overshot wheel still exists), a fulling mill downstream of the village (all that remains is the channel for the undershot wheel) and a windmill (probably a post mill) close to where you are standing, in the garden of the house. Over the past 40 years, the height of the windmill has gradually reduced as the base collapses. It was build of a mix of local brick and clunch (flint with lime mortar) which collapses rapidly if it is not looked after. All three mills were owned by the Bessemer family who lived in Charlton House (Henry Bessemer, the inventor of the Convertor for producing steel was born there on 19th January 1813). Interestingly, the surfaced portion of Windmill Lane has a hardcore base made of industrial furnace slag containing a lot of low-quality iron; where did this come from, since there are no iron founders in the area now.
The cache that you seek is a small black bison zip-tied to a large tree. From the GZ you have an uninterrupted view of the Offley ridge. Please do not drive up the road as it is today a long private drive, although it is also a public right of way. Park safely in the village and walk. This route is popular with dog walkers, joggers, cyclists and the occasional horse rider so it can be busy and a modicum of stealth is needed.