This paper discusses the unique roles played by Ion Cyclotron Resonance Heating (ICRH) in the preparation, formation and sustainment of internal transport barriers (ITBs) in high fusion performance JET optimised shear experiments using the Mk. II poloidal divertor. Together with Lower Hybrid Current Drive (LHCD), low power ICRH is applied during the early ramp-up phase of the plasma current, 'freezing in' a hollow or flat current density profile with q(0)>1. In combination with up to ~20MW of Neutral Beam Injection (NBI), the ICRH power is stepped up to ~6MW during the main low confinement (L-mode) heating phase. An ITB forms promptly after the power step, revealed by a region of reduced central energy transport and peaked profiles, with the ion thermal diffusivity falling to values close to the standard neo-classical level near the centre of both D-D and D-T plasmas. At the critical time of ITB formation, the plasma contains an energetic ICRF hydrogen minority ion population, contributing ~50% to the total plasma pressure and heating mainly electrons. As both the NBI population and the thermal ion pressure develop, a substantial part of the ICRF power is damped resonantly on core ions (ω = 2 ωcD = 3ωcT) contributing to the ion heating. In NBI step-down experiments, high performance has been sustained by maintaining central ICRH heating; analysis shows the efficiency of central ICRH ion heating to be comparable with that of NBI. The highest D-D fusion neutron rates (RNT=5.6x1016 s-1) yet achieved in JET plasmas have been produced by combining a low magnetic shear core with a high confinement (H-mode) edge. In D-T, a fusion triple product niTiτE = (1.2±0.2) x1021 m-3 keVs was achieved with 7.2 MW of fusion power obtained in the L-mode and up to 8.2 MW of fusion power in the H-mode phase.