Basically, my duties are to check that there's no fire hazards in the office, check the fire safety equipment is good (prodding the landlord if necessary), and make sure the exits aren't blocked.
In the event of a fire I tell everyone to get out (leaving any stubborn idiots behind - the course emphasised personal safety probably because if a marshal stays behind there's now two people to rescue), make sure doors are closed on the way out, and once at the assembly point count up everyone and report to the Incident Commander (who will in turn report to the fire brigade). Then wait until the incident commander says it's ok to go back in.
The rest of the training had a bit on which fire extinguisher should be used for what - water/foam for solid fuel fires (like burning paper, which a CO2 extinguisher will tend to blow around thus multiplying your problems), CO2 for anything electric (because BZZZERT) - and how to use them - pull pin, aim from about a metre away, squeeze handles until the fire is really out, and don't hold the horn of a CO2 extinguisher unless you want to freeze your hand. Work doesn't have any gas flooding systems to worry about (my previous work had a FM200 system which is merely unhealthy, but some places use CO2 flooding which will suffocate anyone inside), and our office doesn't have a workshop so we don't need any of the more exotic extinguisher types.