  Using an SU(3) flavour symmetry breaking expansion in the quark mass, we
determine the QCD component of the nucleon, Sigma and Xi mass splittings of the
baryon octet due to up-down (and strange) quark mass differences in terms of
the kaon mass splitting. Provided the average quark mass is kept constant, the
expansion coefficients in our procedure can be determined from computationally
cheaper simulations with mass degenerate sea quarks and partially quenched
valence quarks. Both the linear and quadratic terms in the SU(3) flavour
symmetry breaking expansion are considered; it is found that the quadratic
terms only change the result by a few percent, indicating that the expansion is
highly convergent.
