If you didn't actually know that the steps presented a danger you still have a duty under the 1957 Act to ensure that the steps are reasonably safe. Support for this interpretation is found in the 1984 Act. That act extends 'visitors' to include trespassers, but your liability to them, inter alia, is only for dangers which you know or ought to be aware of. The inference is that your duty towards true visitors exists though you do not actually know of the danger and extends to making sure that the premises are reasonably safe for visitors. It may be argued that, in any event, the existence of a crack in the step suggests some defect which should be investigated and is a potential cause of danger.