The following funding bodies supported this work: Biotechnology and Biological Sciences Research Council (BBSRC BB/L00267/1, to LG), Rosetrees Trust (to LG), Barts and The London Charity (417/2235, to LG), EU COFUND (PCOFUND-GA-2013-608765, to LG and GRB). IH is supported by a Medical Research Council (MRC, G0802796) PhD studentship