Since operation commenced with the ITER-like wall (ILW) in JET, which includes tungsten in the divertor structure, core impurity accumulation, has become an issue. The effect is particularly pronounced in hybrid scenario pulses where tailoring of the current profile is used to achieve higher normalised beta and confinement (H98y>1), than in the ELMy H-mode. A causal link is found between the occurrence of low-n MHD and impurity accumulation for many hybrid pulses with the ILW; in this paper these effects are examined and contrasted with results from the previous carbon wall (C­Wall).