Plasmas with the highest deuterium fusion neutron rates (RDD= 5.6 x1016 s-1) yet achieved in JET have been produced by combining a hollow or flat central current profile with a high confinement (H-mode) edge. In these discharges, LHCD and ICRH preheating, applied early in the current ramp-up phase, 'freezes in' a hollow or flat current density profile. When the combined NBI and ICRH heating power is increased, a region of reduced transport and highly peaked profiles forms during the L-mode phase which persists into the later H-mode phase when the fusion reactivity reaches a maximum. Transport analysis shows the formation of a central region of good confinement (the internal transport barrier or ITB) which expands with radial velocity ~ 0.5m s-1. The clearest signatures of this region are large gradients in the ion temperature and toroidal rotation profiles. Ion thermal diffusivities in the central region are of the order of the neo-classical value. The position and rate of expansion of the ITB radius correlate well with the calculated rational q=2 surface. The confinement improvement can also be seen in electron density, and, to a lesser extent, in electron temperature, but not necessarily at the same radial region. The ITB can persist in combination with the edge transport barrier characteristics of the H-mode.