I have done a bit more research and have found that Capt 9.2 of the NHBC Standards applies. I quote from Section D4 (The D stands for 'design'):-
Unless the stability of new or existing slopes has been determined by an Engineer in accordance with Technical Requirements R5 the following maximum gradients apply:
unsupported granular soil should be 5 degrees less than its natural angle of repose
unsupported cohesive soil should not exceed 9 degrees (1:6)

From your point of view this deals with the stability of the ground and not necessarily the useability of the garden but try hitting them with that quote and I suspect you will hear a sharp intake of breath.