Barbury Castle is an Iron Age Hill Fort above Swindon. The Hill Fort has excellent views of the surrounding countryside (on a clear day you can see Uffington White Horse Hill).
Details of Barbury Castle are available on this web site and also this web site.
This Castle is on the Ridgeway footpath and has a car park with a picnic area, toilets, cafe and excellent views. It can get a bit bleak up here so remember a windproof coat.
There is a choice of parking. The main car park is at N51°29.030', W01°46.640'. It is much the nicest place to park since it has excellent views and provides a nice walk over the hill fort to the cache. You could cache and dash at N51°29.167', W01°47.500'.
An Ordnance Survey map for the location is available at this web site..
When you get to the posted coordinates (on the top of this page) then you'll find an information board. Find out when ERIC was found in the format ABCD and when the disc barrow was built in the format EFGH. The physical cache is at N51°29.(A+E)(G)(A+E)', W01°47.(D)(A+E)(E)'
Please check out the Geocaching Association of Great Britain offering support for Geocachers in GB.