  The asymptotic collinear factorisation theorem, which holds for diffractive
deep-inelastic scattering, has important modifications in the sub-asymptotic
HERA regime. We use perturbative QCD to quantify these modifications. The
diffractive parton distributions are shown to satisfy an inhomogeneous
evolution equation. We emphasise that it is necessary to include both the
gluonic and sea-quark t-channel components of the perturbative Pomeron. The
corresponding Pomeron-to-parton splitting functions are derived in the
Appendix.
