The power output of fusion experiments and fusion reactor-like devices is measured in terms of the neutron emission rates which relate directly to the fusion yield rate. The largest fusion power produced in magnetically confined experiments so far was at JET in 1997, when a peak value of 16MW was achieved. Determination of such parameters requires a set of absolutely calibrated neutron detectors. At JET, the Fission Chamber neutron detectors were originally calibrated some 20 years ago by performing a set of in-situ calibrations using neutron sources and the absolute calibration has been maintained since then by cross calibrations against activation system measurements. After this elapsed time and a succession of changes to the internal and external JET structures, the JET Neutron yield calibration needs re-measurement. A new, more detailed, calibration is being provided by means of an engineering programme of development of the robotic tools which will allow safe and accurate deployment of a strong 252Cf source for the measurements. It is led by a scientific programme which seeks to better understand the limitations of the calibration, to optimise the measurements and other provisions, to provide corrections for perturbing factors and to ensure personnel safety and safe working conditions. Much of this work is based on an extensive programme of Monte-Carlo calculations. These include the updating of previous JET models to provide continuity of comparison with previous understanding, the provision of fast models for side effect estimation and the development of a new more detailed JET model which will allow comparisons with the older more homogeneous model while coping with the demands of the new calibration.