The cache is not at the published coordinates. To discover its location you will need to complete all 5 stages of the Exe Valley Railway series of Adventure Labs.
This Adventure takes you on a tour of the lower Exe Valley and highlights several surviving fragments of the now-defunct Exe Valley Railway that exist in the modern landscape. The distance between the southernmost and northernmost stages is about 10km (6 miles). These Labs were set on a bicycle but can be completed on two wheels or four. You could also make two walks from the southern and northern Labs and combine these with other caches in the area.
The location of the bonus cache will be revealed once all 5 Labs are completed. There is no parking at this cache but you will find a small roadside layby about 100m to the north east.
To play this series of Adventure Labs visit this link.