Steve, I am not aware of any specific regs on aircon ducting maintenance & cleaning. However, if you consider what will accumulate within the exhaust ducting (skin, rodent poo, cotton/wool/man made fibres etc) and to a lesser extent the supply side, anyone opening the ducting for any reason would most likely be exposed to a load of very nasty stuff as well as probably spreading it back in to the working environment. On that basis, COSHH will apply and the appropriate employers must take the required action. A competent HAV engineer should be fully aware of what's needed and be able to do the job with the health risks being adequately controlled. A basic metal basher may approach the job in a less well managed way. The wet parts (if any) of course will need to be formally managed to prevent bacterial build-up.

Just a quick note to say I've changed my user name from that one above (starts GC-) to this. Yep made a newbie mistake. Advice still stands.