An incident occurred involving a Genie Z135/70 mobile elevating work platform boom failure at a Tata Steel site in Port Talbot. While attempting to slew the basket at full reach, the basket unexpectedly started tilting towards the ground. The operator was unable to correct the tilt from controls in the basket or on the ground. As a result, all Genie Z135/70 MEWPs are prohibited from hire for use on or around Network Rail until the root cause is identified and remedial actions are taken.