A non-steady-state orbit-following Monte-Carlo code has been developed. As part of the verification and validation of the code, calculations of time dependent neutron rates have been made for a specific shot in the Mega Amp Spherical Tokamak (MAST) using three-dimensional fields representing vacuum resonant magnetic perturbations (RMPs) and toroidal field (TF) ripples. The calculation results approximately agree with the experimental data.