Timothy, thanks for your thoughts. You have perfectly portrayed my fears in that we could be prosecuted for flying near anything that someone had erected and the meaning is definitely not "a building".
Greeners, that was also my interpretation, but as Timothy pointed out, dictionaries do not agree. Is the law defined by dictionary or intended meaning?
The main reason for asking the question is that I often fly very low along a sea wall. The purpose is to keep the wall below the aircraft, whilst maintaining an exact altitude (OK, within +/- about 10 feet).
I cannot, no matter how much I drink on a rainy Sunday, see a runway classed as a structure.