Transport analysis of high performance JET plasmas with optimised magnetic shear (OMS) has revealed many interesting features which can not be explained by the existing JET empirical transport model [1]. TRANSP analysis shows that transport coefficients in OMS plasmas are often reduced in the plasma core [2] to the level of ion neoclassical transport. TRANSP analysis and predictive modelling with JETTO show that this region of improved confinement appears near the plasma centre and then expands outwards in a way which does not follow either the evolution of the region with the negative magnetic shear or the propagation of the region with a large shear in plasma rotation. The best agreement with experiment has been achieved by using a transport model which combines the effect of a long wavelength decoupling due to small magnetic shear with its suppression by strong rotational shear. Predictive modelling of some of the characteristic JET OMS plasmas gives quite good agreement between such a model and the experimental data.