A Hamiltonian guiding centre particle following code has been developed to study the fast particle motion in the presence of arbitrary time-dependent electromagnetic perturbations. In conjunction with an MHD stability code, this code was used to analyse TAE/KTAE-induced α-orbit diffusion and α-losses in JET plasmas. Resonant α-orbits are studied below and above the stochasticity thresholds, in the presence of single or several TAE and KTAE modes. Monte Carlo randomised ensembles of α-particles in the presence of finite-amplitude TAE/KTAE modes are followed and estimates for the stochastic diffusion coefficients are obtained. Generalisation of the method towards the self-consistent wave-particle evolution is described.