Neil, your story prompts me to tell another - it's not actually MY story although I was there - but the IO who's story it is isn't online so I hope she'll forgive me for stealing it.
So there we are - 3 little bike novice marshals (well maybe not so little). 2 of us have been doing cars, so we're more or less ok with the basics of the anatomy of a fire extinguisher.
The other guy has never marshalled before. Our IO diligently explains the safety rules and then moves on to the extinguisher and gives a superb 10 minute session on extinguisher operation and testing.
Grand!
Conversation then ensues about what everyone does for a living. 3rd marshal is asked what he does. "I'm a fireman"
DOH!
(he couldn't fault her extinguisher lecture though so all's well that ends well!
)