You are looking for a disguised micro-container.
Please keep dogs on leads as there are often sheep grazing at the cache location.
This is one of my favourite spots. I discovered it one day when I took a long walk out from Cardiff to Cafn Mably along the banks of the River Rhymney. Sometimes we make it a shorter walk by parking in Old St Mellons and accessing the trailhead coordinates from there. If you choose to walk this way the first part of the walk will be next to a new housing development. After passing under the M4 you will have a feeling of entering the countryside. The footpath enters a wonderful field where sheep graze and there are lots of old trees. I often stop and eat my lunch around here.
The nearby Cefn Mably House is of historical significance. Parts of it go back to the sixteenth century. In the mid-1900s it was used as a tuberculosis hospital. After that it was left empty and badly vandalised. Fortunately it has been saved and turned into luxury apartments and forms part of a gated community. There are however still rights of way through the community complex.