Tried to visit this hillfort today while out walking around the Coates area. Have to report defeat on this one though note thesweetcheat managed it. We were heading for the 'source' of the Thames which seems significantly very close to the hillfort. Entering by a gate through woodland (no 'private' notices though it felt private) we came up against a deep walled ditch - so turned back. The OS map shows Trewsbury House inside the hillfort: http://www.britishlistedbuildings.co.uk/en-129296-trewsbury-house-coates/osmap
Quite unusual and made us less inclined to press on - could also hear what sounded like a flock of geese ...
We found the 'source' of the Thames marked by a large granite stone - now a dry spring.
Passed this low-lying fort after a trip to Windmill Tump today. Approached from the west along the disused Thames and Severn Canal. The west side of the fort (which is all I saw as the rest is on private land) appears to be a steep bank, but as the canal ran alongside it's difficult to know what was fort and what was canal engineering.
Of interest is the fact that the spring source of the River Thames is at the southern tip of the fort. As the Thames was venerated by the Iron Age people (witness the number of swords and other offerings thrown in further downstream), it may be that this had some significance in the siting of the fort.