  The SU(3) flavour symmetry breaking expansion in up, down and strange quark
masses is extended from hadron masses to meson decay constants. This allows a
determination of the ratio of kaon to pion decay constants in QCD. Furthermore
when using partially quenched valence quarks the expansion is such that SU(2)
isospin breaking effects can also be determined. It is found that the lowest
order SU(3) flavour symmetry breaking expansion (or Gell-Mann-Okubo expansion)
works very well. Simulations are performed for 2+1 flavours of clover fermions
at four lattice spacings.
