A gas turbine engine comprising: a first shroud block including a mounting slot; a blade outer air seal supported within the mounting slot; a seal disposed within the mounting slot providing a seal between the blade outer air seal and the mounting slot; and a first anti-rotation tab attached to the first shroud block within the mounting slot for constraining movement of the seal within the mounting slot, characterised in that: the first anti-rotation tab is disposed at a first end of the mounting slot, with a second end of the mounting slot distal from the first end not including an anti-rotation tab such that the seal may be slid from the second end into abutment with the first anti-rotation tab at the first end of the mounting slot; and in that the gas turbine engine includes a plurality of shroud blocks with a corresponding plurality of anti-rotation tabs, each disposed at the first end of each mounting slot, such that the seal disposed within the mounting slot of the first shroud block is contained at the first end of the mounting slot by the first anti-rotation tab disposed within the first shroud block and at the second end of the mounting slot by a second anti-rotation tab disposed within a corresponding second shroud block. a first shroud block including a mounting slot; a blade outer air seal supported within the mounting slot; a seal disposed within the mounting slot providing a seal between the blade outer air seal and the mounting slot; and a first anti-rotation tab attached to the first shroud block within the mounting slot for constraining movement of the seal within the mounting slot, characterised in that: the first anti-rotation tab is disposed at a first end of the mounting slot, with a second end of the mounting slot distal from the first end not including an anti-rotation tab such that the seal may be slid from the second end into abutment with the first anti-rotation tab at the first end of the mounting slot; and in that the gas turbine engine includes a plurality of shroud blocks with a corresponding plurality of anti-rotation tabs, each disposed at the first end of each mounting slot, such that the seal disposed within the mounting slot of the first shroud block is contained at the first end of the mounting slot by the first anti-rotation tab disposed within the first shroud block and at the second end of the mounting slot by a second anti-rotation tab disposed within a corresponding second shroud block. the first anti-rotation tab is disposed at a first end of the mounting slot, with a second end of the mounting slot distal from the first end not including an anti-rotation tab such that the seal may be slid from the second end into abutment with the first anti-rotation tab at the first end of the mounting slot; and in that the gas turbine engine includes a plurality of shroud blocks with a corresponding plurality of anti-rotation tabs, each disposed at the first end of each mounting slot, such that the seal disposed within the mounting slot of the first shroud block is contained at the first end of the mounting slot by the first anti-rotation tab disposed within the first shroud block and at the second end of the mounting slot by a second anti-rotation tab disposed within a corresponding second shroud block.