A locking mechanism for releasably locking a lock bolt against axial movement, the mechanism comprising an axially moveable member, arranged to move axially with the lock bolt biasing means, arranged to bias the piston towards the lock bolt; rotation means in threaded engagement with the piston such that rotation of the rotation means causes axial movement of the piston, and a solenoid assembly arranged to prevent rotation of said rotation means in a locked state, the solenoid assembly comprising: a solenoid plunger having a solenoid tip and a solenoid arranged to cause movement of the solenoid plunger relative to the rotation means, the solenoid assembly arranged such that in a locked state the solenoid causes the solenoid tip to engage with the rotation means such as to prevent rotation thereof, thus preventing axial movement of the piston.