  We discuss leading isospin breaking effects in the hadronic vacuum
polarisation required for the investigation of the hadronic contribution to
$(g-2)_{\mu}$. The calculation proceeds by expanding the relevant correlation
functions around the isosymmetric limit. Isosymmetric observables are evaluated
on gauge ensembles with $N_{f}=2+1$, $O(a)$ improved Wilson fermions and open
boundary conditions generated by the CLS effort. Particular emphasis is placed
on the relevant quark-disconnected diagrams required for a complete treatment
of leading isospin breaking effects in the valence quark sector. We provide a
detailed discussion of the renormalisation of the vector current in QCD+QED
taking operator mixing into account.
