  Diffractive parton distributions of the proton are determined from fits to
diffractive data from HERA. In addition to the twist--2 contribution, the
twist--4 contribution from longitudinally polarised virtual photons is
considered, which is important in the region of small diffractive masses. A new
prediction for the longitudinal diffractive structure function is presented
which differs significantly from that obtained in the pure twist--2 analyses.
