An employer shall ensure that a supporting structure is -
(a) suitable and of sufficient strength and rigidity for the purpose for which it is being used,
(b) in the case of a mobile structure, prevented by appropriate devices from moving inadvertently during work at height,
(c) in a case other than a mobile structure, prevented from slipping by secure attachment to the bearing surface or to another structure, by provision of an effective anti-slip device, or by other means of equivalent effectiveness,
(d) stable while being erected, used and dismantled, and
(e) when altered or modified, so altered or modified as to ensure that it remains stable.