The polarisation of an electromagnetic wave travelling through an optically active and birefringent medium such as a magnetised plasma changes due to both Faraday rotation and Cotton-Mouton effects. From the latter, in certain conditions, one can obtain information on the line-integrated plasma density, which is usually measured by the interferometer, in terms of the Cotton-Mouton phase shift. The work reports the comparison of the line-integrated plasma density derived from interferometer and polarimeter diagnostics measurements with the purpose of testing different models to recover the line integrated plasma density from the measured Cotton-Mouton phase shift. A systematic analysis of the quality of the Cotton-Mouton measurements for a wide range of plasma conditions at JET (different regimes of temperature, density and toroidal magnetic field) is presented for the first time. The purpose of the work was to identify a suitable model for the line-integrated plasma density from polarimeter. These new measurements could be used to alleviate the problem of "fringe jumps" in the interferometer, which poses serious difficulties for the plant safety and real-time control of many JET experiments. Many of the reported results will also contribute and support the design of ITER interferometer/polarimeter.