  We present the results of a QCD fit to global data on deep-inelastic
polarised lepton-hadron scattering. We find that it is possible to fit the data
with strongly broken SU(2) flavour for the polarised sea densities. This can
easily be tested in $W$ production at polarised RHIC. The data fails to pin
down polarised singlet sea quark and gluon densities. We explore the
uncertainties in detail and show that improvement in statistics, achievable at
polarised HERA for measurement of A_1 at moderately low values of x, have large
payoffs in terms of the improvement in measurement of gluon and sea quark
densities.
