The asymptotic dynamics of the spin autocorrelation function is studied in the Griffiths phase of bond-dilute Ising and Heisenberg ferromagnets by Monte Carlo simulation, for simple relaxational dynamics (model A). Systems above, at and below the percolation threshold are studied. Relaxation is non-exponential in all cases. Agreement with theoretical predictions based on clustering arguments is excellent for the Heisenberg systems, less so for the Ising systems.