\section{Acknowledgements}



Material used in this work has been provided by the Coordination Unit 5 (CU5) of the Gaia Data Processing and Analysis Consortium (DPAC). They are gratefully acknowledged for their contribution.

HCC, STH, GG, NAW, LW are members of the Gaia Data Processing and Analysis Consortium (DPAC) and this work has been supported by the UK Space Agency.  NB has been supported by the The Gaia Research for European Astronomy Training (GREAT-ITN) network, funded through the European Union Seventh Framework Programme ([FP7/2007-2013] under grant agreement n$\,^{\circ}$ 264895.
