The first resource on scene from each agency will by default become the 'Incident Commander' for that organisation until relieved
Commence a systematic process of scene assessment and management
Look around the scene and make notes
Convey a M/ETHANE message to 999
Coordinate triage and evacuation process
DO NOT start to treat patients, but you may direct walking wounded to a central point away from the scene
DO NOT start triage of patients, but educated guesses on numbers and types - P1s, P2s, P3s, Dead
Assess and manage broader risks
Regular updated METHANE messages, especially with potential and actual hazards