What duty does an occupier owe to an invitee?

The duty that an occupier owes to an invitee was expressed by Willes J. in Indermaur v. Dames (1866), L.R. 1 C.P. 274 at 288 as follows: ... [W]e consider it settled law, that he, using reasonable care on his part for his own safety, is entitled to expect that the occupier shall on his part use reasonable care to prevent damage from unusual danger, which he knows or ought to know....