  We calculate the spin and charge dynamical susceptibilities of a strongly
correlated impurity model in a renormalised perturbation theory.
  The irreducible for vertices for the quasiparticle scattering are deduced
from the renormalised parameters, which have been calculated by fitting of the
low-lying levels of a numerical renormalisation group (NRG) calculation to
those of an effective Anderson model. The susceptibilities are asymptotically
exact in the low frequency limit and satisfy the Korringa-Shiba relation. By
comparing the results with those calculated from a direct NRG calculation, we
show that the renormalised perturbation theory (RPT) description gives a very
good description of spin dynamics for all values of the local interaction U,
not only at low frequencies but over the whole relevant frequency range.
  In the presence of a magnetic field the approach can be generalised using
field dependent renormalised parameters to calculate the transverse and
parallel spin dynamic susceptibilities. The RPT results give accurate results
for the spin dynamics over the whole frequency range and for all values of the
magnetic field.
