Found them! And guess what they're called...
The Trump Manual Blow Horn combines the powerful blast of sound of a gas fog horn with the simplicity of a mouth horn.
Delivers over 100dB with no effort. 70mm diameter. 195mm length.
Bring earplugs. I saw on MOTD that not only do they have an amplified drum/s and coordinated singing, they also have clackers.
Currently researching miniature airhorns for the (2000) away section to sneak in...