If it were me, with my French background, I would use "is" because it's referring to the sentence subject "one in six".
In French, both would be okay, even though one way may be
more right 
For example, we can say
Quote:
|
The majority of the readers agrees with the publisher.
|
(
correct grammar wise since the subject of the sentence is "the majority")
as well as
Quote:
|
The majority of the readers agree with the publisher.
|
(
okay since the theme of the sentence clearly states there are more than one reader).
Now French and English grammar rules can sometimes be very close, however I won't say that what applies to one stays true for the other
