  We consider the factorisation of one-loop amplitudes at complex kinematic
points. By determining the terms that are absent for real kinematics, we can
construct a recursive ansatz for the purely rational pieces of one-loop
amplitudes in massless theories. We illustrate this method by verifying the
Bern et.al. n-point ansatze for the single-minus one-loop amplitudes in
Yang-Mills theory and by constructing the scalar contribution to the one-loop
five graviton MHV scattering amplitude.
