“1. Preliminary (1) The rules enacted by the two next following sectionsshall have effect, in place of the rules of the common law, to regulate the duty which an occupier of premises owes to visitors in respect of dangers due to the state of the premises or to things done or omitted to be done on them… 2. Extent of occupier’s ordinary duty (1) An occupier of premises owes the same duty, the “common duty of care”, to all his visitors, except in so far as he is free to and does extend, restrict, modify or exclude his duty to any visitor or visitors by agreement or otherwise. (2) The common duty of care is a duty to take such care as in all the circumstances of the case is reasonable to see that the visitor will be reasonably safe in using the premises for the purposes for which he is invited or permitted by the occupier to be there. “(3) The circumstances relevant for the present purpose include the degree of care, and of want of care, which would ordinarily be looked for in such a visitor, so that (for example) in proper cases— (a) an occupier must be prepared for children to be less careful than adults…”