  Accurate modelling of the T2K neutrino flux is crucial for a better
understanding of neutrino interactions at the near and far detectors. Most of
T2K neutrinos are created through in-flight decays of unstable hadrons,
produced by interactions of 31 GeV/c protons in a long graphite target (90 cm).
External hadron production data is used for correcting the flux model. The
analysis presented here uses a new NA61 dataset, collected in 2009 using the
full length replica of the T2K target. The preliminary results suggest a
reduction of the hadronic interaction component of the neutrino flux
uncertainty by ~50%, to errors of less than 5%.
