The ELM-free hot-ion H-mode regime had been investigated in the JET Mk II divertor configuration over a wide range of plasma conditions in deuterium (DD) discharges prior to the start of the recent DT campaign (DTE1). Experiments in DD with neutral beam (NB) heating alone (up to 19MW) and also with supplementary RF heating (up to 25MW combined heating power) were performed, at plasma currents up to 4.2MA and toroidal field up to 3.8T. Characteristics of the regime such as: confinement and DD fusion performance [1]; MHD stability (sawteeth, outer modes and ELMs) [2]; and influence of particle recycling [3], were all extensively investigated in DD. The optimum experimental conditions for exploitation in DT of the ELM-free regime could thus be defined [4], except the prescription for obtaining the ideal D:T fuel mix in the plasma. Preparatory DT experiments described in this work were designed to address this question and their interpretation has also yielded new insight into the contribution of recycled particles to fuelling (even in the presence of strong NB injection), and also the relative transport of tritium and deuterium within the plasma ("particle mixing").