With installation of the ITER-like Wall in JET a major diagnostic upgrade to measure the neutral gas pressure and composition in the sub-divertor region has been completed, to characterise retention and outgassing of the new metallic first wall. The upgrade includes two new magnetically shielded systems consisting of sensitive capacitance manometers and residual gas analysers, both capable of providing data during plasma operation. These enable absolute pressure and gas composition measurements (pressure range: 10-5 ­ 10-1 mbar, mass range: 1­200amu respectively) and have been used to characterise the neutral gas behaviour under various plasma conditions.