  We study perturbations in the multi-field axion Nflation model, taking
account of the full cosine potential. We find significant differences with
previous analyses which made a quadratic approximation to the potential. The
tensor-to-scalar ratio and the scalar spectral index move to lower values,
which nevertheless provide an acceptable fit to observation. Most
significantly, we find that the bispectrum non-gaussianity parameter f_NL may
be large, typically of order 10 for moderate values of the axion decay
constant, increasing to of order 100 for decay constants slightly smaller than
the Planck scale. Such a non-gaussian fraction is detectable. We argue that
this property is generic in multi-field models of hilltop inflation.
