  We study the topological susceptibility, chi, in QCD with two quark flavours
using lattice field configurations that have been produced with an O(a)
improved quark action. We find clear evidence for the expected suppression at
small quark mass, and examine the variation of chi with this mass. The
resulting estimate of the pion decay constant, f_pi = 105 +/- 6 ^{+18}_{-10}
MeV, is consistent with the experimental value of ~93 MeV. We compare chi to
the large-N_c prediction and find consistency over a large range of quark
masses. We discuss the benefits of the non-perturbative action improvement
scheme and of the stategy of keeping the lattice spacing (nearly) fixed as the
quark mass is varied. We compare our results with other studies and suggest why
such a quark mass dependence has not always been seen.
