The Fibre Optic Current Sensor (FOCS) is a diagnostic system to be installed in ITER to perform plasma current measurements during long plasma discharges under intense nuclear radiation. The sensor performance must be experimentally assessed at ITER representative conditions. JET is a unique machine where this task can be performed. We have installed polarisation detection based FOCS systems at JET and performed measurements in various machine operating scenarios at currents up to 3.5 MA. The frequency analysis of the data was performed to investigate the sensitivity of FOCS to mechanical perturbations. A significant part of the noise originates from the optical link between the sensing fibre and the polarisation analyser. Operation with a Faraday mirror reduces this effect. It is confirmed that the FOCS system has performance characteristics compatible with the ITER requirements.