"An unqualified or unskilled person would not have foreseen that removing the wall involved a risk of harm. And, although the concept of a 'reasonable person' is not wholly objective, it is not appropriate to apply the test of a 'reasonable professional person', such as an engineer, doctor or lawyer. A 'reasonable person' may be considered to have knowledge of particular facts, but should not be taken to have professional expertise, professional skill or professional knowledge."