  We present a new calculation of the K->pi semileptonic form factor at zero
momentum transfer in domain wall lattice QCD with Nf=2+1 dynamical quark
flavours. By using partially twisted boundary conditions we simulate directly
at the phenomenologically relevant point of zero momentum transfer. We perform
a joint analysis for all available ensembles which include three different
lattice spacings (a=0.09-0.14fm), large physical volumes (m_pi*L>3.9) and pion
masses as low as 171 MeV. The comprehensive set of simulation points allows for
a detailed study of systematic effects leading to the prediction
f+(0)=0.9670(20)(+18/-46), where the first error is statistical and the second
error systematic. The result allows us to extract the CKM-matrix element
|Vus|=0.2237(+13/-8) and confirm first-row CKM-unitarity in the Standard Model
at the sub per mille level.
