Abstract: Apparatus (3) for detecting a leak of molten plastics material in a hot runner mould (1) comprises a detecting circuit (28) which in turn comprises a plurality of tubular detecting elements (30) connected in series by connecting conduits (32). The detecting circuit (28) is located in galleries (25) in the hot runner mould (1) with the tubular detecting elements (30) located adjacent locations in the galleries (25) where leaks are likely to occur. A compressed air source (40) delivers compressed air at a regulated pressure through the detecting circuit (28) at a controlled flow rate controlled by a flow controller (45). A first pressure sensor (43) at the upstream end of the detecting circuit (28) and a second pressure sensor (44) at a downstream end of the detecting circuit (28) monitors the pressure upstream of the detecting circuit (28).