The way I heard the story (from a senior PSA guy) was that the design authority for the road (yes, PSA) read the rainfall figures, which were in millimetres, as inches.
Yhey did the same at Odiham many moons ago and built a shelter outside Ground Equip for the powersets, they mixed up Feet and Metres resulting in a shed that was about the depth of a powerset, but you could put a fully raised safety raiser under and still not reach the roof.