The plug (16) includes a longitudinal axis (50) and a keyway (24).

A key channel in the head assembly is adapted for receiving a key and into which the indexing wheel projects.

The keyway (24) is adapted to receive at least a first user key (30) and a second user key (62).

A plurality of key actuated moveable blocking elements block the rotation of the cylinder core unless a correct key is inserted in the key-way.

The double entry keyway means that the key can be inserted either way up, for the user’s convenience.

A returning means (50) is arranged to mechanically co-operate with the key and the actuator to move said actuator to a blocking position in response to withdrawal of the key from said key-way.