  We numerically solve the Klein-Gordon equation at second order in
cosmological perturbation theory in closed form for a single scalar field,
describing the method employed in detail. We use the slow-roll version of the
second order source term and argue that our method is extendable to the full
equation. We consider two standard single field models and find that the
results agree with previous calculations using analytic methods, where
comparison is possible. Our procedure allows the evolution of second order
perturbations in general and the calculation of the non-linearity parameter
f_NL to be examined in cases where there is no analytical solution available.
