Buxus sempervirens - circular brown die-outs (disease issue?)

Hello green thumbs!

Have a question I'm hoping you can help with, regarding the Buxus hedge pictured below:

http://pics.livejournal.com/nobudsnoglory/pic/0004wky6/s320x240

http://pics.livejournal.com/nobudsnoglory/pic/0004x07c/s320x240

http://pics.livejournal.com/nobudsnoglory/pic/0004yacy/s320x240

As you can see, it's suffering from some kind of strange brown, die-out in an almost eerily circular repetition along its length. This problem was first noticed in the hedge a few months ago, when it was initially took for someone lopping a part out with scissors. Recently, these brown patches have begun appearing down the length of the hedge, interrupting what has been until now a wonderful green accent to the garden...

We are at a loss to understand what it is, or what to do about it. Do any of you have any ideas of disease issues that may have caused these symtoms, and - perhaps most importantly - what we could do about it?

Thanks in advance!

 

Check to see if these help:
http://www.rhs.org.uk/advicesearch/profile.aspx?PID=96
http://www.rhs.org.uk/Learning/Research/Projects/cylindrocladium.htm