  We study the topological susceptibility, chi, in QCD with two quark flavours
using the lattice field configurations recently produced by the UKQCD
collaboration. 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 +/- 5 +18/-10 MeV, is
consistent with the experimental value of approximately 93 MeV. We compare chi
to the large-N_c prediction and find consistency over a large range of quark
masses. The benefits of the non-perturbative action improvement scheme and the
matched lattice spacings between simulation ensembles are discussed. We compare
our results with other studies and suggest a reason why such a quark mass
dependence has not been previously seen.
