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 around a rotation axis 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, characterised in that: the drive member comprises a drive member engaging structure extending along an arc of constant radius of curvature with respect to said rotation axis, the drive member engaging structure being configured to engage with a linear coupling member engaging structure formed in the coupling member. 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 around a rotation axis 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, characterised in that: the drive member comprises a drive member engaging structure extending along an arc of constant radius of curvature with respect to said rotation axis, the drive member engaging structure being configured to engage with a linear coupling member engaging structure formed in the coupling member. 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 around a rotation axis 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, characterised in that: the drive member comprises a drive member engaging structure extending along an arc of constant radius of curvature with respect to said rotation axis, the drive member engaging structure being configured to engage with a linear coupling member engaging structure formed in the coupling member. the drive member comprises a drive member engaging structure extending along an arc of constant radius of curvature with respect to said rotation axis, the drive member engaging structure being configured to engage with a linear coupling member engaging structure formed in the coupling member.