We have been told that our cooling tower has the legionnella bacteria and this will have been released into the surrounding area.
No. The escape or release of a biological agent must only be reported if it causes severe human infection or illness. In practice, this means a biological agent in hazard groups 3 or 4 in Part V, Schedule 3 of the Control of Substances Hazardous to Health Regulations 2002 (COSHH). Legionnella is in group 2, so it is not reportable.