This cache series is not designed to be easy. The caches themselves are all quite easy to find, but determining the coordinates of each will require some puzzle-solving effort.
When you visit the cache, be sure to take a printed copy of the bonus puzzle sheet [PDF] so that you can decipher the coordinates of the secret bonus cache! Alert: You are about to download a file that contains further details needed to find the secret bonus geocache. As the cache owner, I represent that this file is safe to download although it has not been checked by Groundspeak or by the reviewer for possible malicious content. Download this file at your own risk.
Go to this web page and inspect the text. Somewhere, hidden within it, are the co-ordinates of the cache.
Getting to the cache itself may require either long arms, a "boost" from a friend, or a little bit of climbing, but nothing that should overstretch anybody in reasonable physical fitness. The cache itself is alongside a footpath that attracts a lot of cycle traffic, so be prepared to use a little stealth if you attract the attention of passer-by: you shouldn't need much, though, because there's no reason to stop here other than to hunt for the geocache!
Note that parking on the roads nearest the cache are restricted to residents only. There are a small number of public parking spaces outside the shops on the other side of Headington Roundabout, and a small network of pedestrian subways provide access to and from the sides of the roundabout.
Need a hint to help decipher the coordinates? You might find what you're looking for on the supporting web page.