Viewing panel unit and structures comprising the viewing panel unit

A viewing panel unit, comprising:

• a first panel and a second panel;
• a drive mechanism for moving the first panel relative to the second panel;
• and
• an abutment member mounted so as to have a fixed spatial relationship relative to the second panel, wherein:
• the first panel comprises a coupling member;
• the drive mechanism comprises a drive member configured to engage with the coupling member such that rotation of the drive member imparts a linear motion to the first panel via the coupling member;
• and
• the drive mechanism is configured such that a first impact surface of the drive member is brought into contact with a surface of the abutment member to stop the drive member when the drive member is rotated through a maximum angle in a first sense,
• wherein the drive member comprises a drive member engaging structure extending along an arc of constant radius of curvature, the drive member engaging structure being configured to engage with a linear coupling member engaging structure formed in the coupling member to move the first panel laterally to the axis of rotation of the drive member, and
• wherein the first impact surface is formed on a surface lateral of the drive member engaging structure.