A diverter for diverting a fluid from a riser in a drilling system. The diverter includes a diverter support housing comprising a suspension structure configured so that the diverter support housing is suspendable from a drilling rig, and a main passage arranged to extend from an uppermost end of the diverter support housing to a lowermost end of the diverter support housing. The main passage is configured to have a drill string extend therethrough. A diverter housing is arranged in the main passage. An annular packer element is mounted within the diverter housing. An actuator is mounted within the diverter housing. The actuator is configured to force the annular packer element into a sealing engagement with the drill string. A seal locking mechanism is configured to retain a tubular sealing element in the diverter housing adjacent to the annular packer element.