Seeding of impurities into JET resulted in a pronounced modification of plasma confinement properties. In order to analyse the underlying mechanisms a predictive transport modelling has been performed by means of two approaches. (i) The Chalmers advanced fluid model for temperature gradient driven instabilities is applied to determine all transport coefficients necessary for transport modelling. In the present code computations the radial profiles of the particle densities and temperatures are computed using experimentally measured sources. The computations gives good agreement of the temperature profiles with the measured ones. (ii) The J¸lich transport model, used earlier to simulate the plasma evolution in TEXTOR radiative improved mode, has been amended by a proper description of trapped electron transport, which is important at low collisionality typical for JET. With this model incorporated the transport code RITM, which self-consistently computes particle sources due to plasma recycling and impurity ionisation, reproduces well the peaking in density and pressure profiles caused by impurity seeding.