An apparatus for use in a fluid conduit, the apparatus comprising: a plurality of segments arranged along a longitudinal axis, each segment comprising an extendable structure configured for extension in a lateral direction relative to the longitudinal axis; wherein the apparatus is configured for the sequential actuation of lateral extension of the extendable structures of at least two segments; characterised in that: the apparatus is configured for the sequential actuation of lateral extension of the extendable structures of at least two segments by application of an axial actuation force applied at or through opposing axial ends of each segment; wherein the apparatus comprises a seal structure configured to be axially compressed to a sealing configuration by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; wherein the apparatus further comprises a substantially incompressible deformable support or filler material disposed around or adjacent the extendable structure of at least one segment, the deformable support or filler material configured to be deformed by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; and wherein, in the sealing configuration, the seal structure is supported by the extendable structures of the at least two segments. a plurality of segments arranged along a longitudinal axis, each segment comprising an extendable structure configured for extension in a lateral direction relative to the longitudinal axis; wherein the apparatus is configured for the sequential actuation of lateral extension of the extendable structures of at least two segments; characterised in that: the apparatus is configured for the sequential actuation of lateral extension of the extendable structures of at least two segments by application of an axial actuation force applied at or through opposing axial ends of each segment; wherein the apparatus comprises a seal structure configured to be axially compressed to a sealing configuration by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; wherein the apparatus further comprises a substantially incompressible deformable support or filler material disposed around or adjacent the extendable structure of at least one segment, the deformable support or filler material configured to be deformed by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; and wherein, in the sealing configuration, the seal structure is supported by the extendable structures of the at least two segments. the apparatus is configured for the sequential actuation of lateral extension of the extendable structures of at least two segments by application of an axial actuation force applied at or through opposing axial ends of each segment; wherein the apparatus comprises a seal structure configured to be axially compressed to a sealing configuration by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; wherein the apparatus further comprises a substantially incompressible deformable support or filler material disposed around or adjacent the extendable structure of at least one segment, the deformable support or filler material configured to be deformed by the sequential actuation of the lateral extension of the extendable structures of the at least two segments; and wherein, in the sealing configuration, the seal structure is supported by the extendable structures of the at least two segments.