The authors present experimental results for the ionisation of excited hydrogen atoms, with initial principal quantum number n0 in the range 32<or=n0<or=48, by low frequency microwaves in the scaled frequency range 0.05( Omega (0.2. They compare these with results of a classical Monte Carlo calculation showing that there is structure in the experimental ionisation curves of quantal origin. A further comparison with a simple one-dimensional quantal theory, using an adiabatic basis, shows that the observed structure can be explained in terms of resonances between a few adiabatic states. A very simple two-state model is shown to possess all the relevant features of the system. Further analysis of this one-dimensional model shows that when the field frequency tends to zero the static field ionisation limit is approached through infinitely many such resonances, at Omega approximately=1/p, of width O(e-p), p being a large integer.