When you turn on the AC in my office it smells like someone in the system is hanging old socks in front of the air flow. What's this all about then?

Someone's done a big poo in it

You need to get it serviced as the filters will need cleaning and the system will need de-bugging.
But basically as the system works it drops moisture out of the air and this is collected and taken to drain but it does cause bugs to grow on th esystem and they should be serviced every six months.

Build up of bacteria…get it serviced and de-bugged…otherwise it will continue to pump this bacteria around the air for you to breath.

It is serviced regularly but still reeks when it is fired up. Lessens after a bit but I think the stale water theory probably holds the most weight if it is not draining properly.

Have a read:

http://www.legionellacontrol.com/legionnaires-disease.htm

& here:

http://www.independent.co.uk/news/uk/crime/council-charged-with-manslaughter-after-legionnaires-outbreak-569593.html

Then moan at your boss to sort it out.

Anyone at your work gone off with the flu recently? because if they have:

If you suspect that you may have got the illness as a consequence of your work then you are advised to report this to your manager, as well as your health and safety representative and occupational health department, if you have one. In the UK there is a legal requirement for employers to report cases of Legionnaires' disease that may be acquired at their premises to the Health and Safety Executive.

Where I used to work we had a number of climatic chambers, and over a 5 year period they found legionaries bacteria 3 times. Not difficult or expensive to sort out, but you have to do it.