Analysis and modelling of fast-ion deuterium-α (FIDA) spectroscopy data from both MAST and MAST-U was performed and is described, along with description of the setup of the diagnostic for the first experimental campaign on MAST-U. Analysis of archival MAST data featuring sawtooth crashes show that, while insufficient to distinguish between 3 different ways of modelling the effect of sawtooth crashes on the fast ions, can observe significant redistribution of the fast ions due to the crashes. FIDA data was used to add to a bank of evidence showing problems with the configuration of the neutral beam injection system, which was later confirmed. Analysis of the effect of a large fishbone instability on plasma rotation shows that the large deceleration of the plasma is unlikely to be due to forces induced by a radial inflow of bulk ions, itself induced by a radial outflow of fast ions. An attempt is made to utilise anomalous diffusion in order to examine the effect of Toroidal Alfven Eigenmode (TAE)s and fishbones on the fast ions. Investigation into the effect of locked modes and long-lived mode (LLM)s show severe redistribution due to the locked mode, with correlations made between the dynamics of the locked mode and the FIDA emission. Investigation of the LLM show significant reduction of FIDA emission in core channels, suggesting localised redistribution. Future integration of multiple fast-ion diagnostics including the Solid State Neutral Particle Analyser will allow for a deeper understanding of fast ion/instability interaction.