  The semileptonic decays of polarised top quarks are analysed for a general
chirality-conserving tbW vertex. We calculate double differential distributions
for the charged lepton and the neutrino to order \alpha_s in the QCD coupling.
We present these QCD corrections in terms of compact parameterisations that
should be useful for the future investigation of the structure of the top decay
vertex on the basis of large data samples.
