A method for setting a packer in a wellbore, the method comprising: providing a packer having a packer mandrel with a seal assembly (100,102,104) and a piston slidably disposed thereabout; running the packer into the wellbore; resisting a force acting on the piston that is generated by a pressure difference between pressure in the wellbore and pressure in a chamber defined between the piston and the packer mandrel by restraining movement of the piston toward the seal assembly with a frangible member; characterised in that the method additionally comprises: decoupling a collet assembly disposed about the packer mandrel from a sleeve slidably disposed about the packer mandrel with a liner in the wellbore; shifting the sleeve relative to a packer mandrel extension toward the piston by the movement of the packer causing the sleeve to contact a liner top of the liner; aligning a fluid port of sleeve with a fluid port of packer mandrel extension, allowing wellbore pressure to enter a fluid pathway and act on the lower surface of piston; breaking the frangible member coupling the piston and the packer mandrel; and responsive to the pressure difference, longitudinally shifting the piston relative to the packer mandrel toward the seal assembly (100,102,104), to operate the seal assembly (100,102,104) from a running position to a radially expanded sealing position, thereby setting the packer. providing a packer having a packer mandrel with a seal assembly (100,102,104) and a piston slidably disposed thereabout; running the packer into the wellbore; resisting a force acting on the piston that is generated by a pressure difference between pressure in the wellbore and pressure in a chamber defined between the piston and the packer mandrel by restraining movement of the piston toward the seal assembly with a frangible member; characterised in that the method additionally comprises: decoupling a collet assembly disposed about the packer mandrel from a sleeve slidably disposed about the packer mandrel with a liner in the wellbore; shifting the sleeve relative to a packer mandrel extension toward the piston by the movement of the packer causing the sleeve to contact a liner top of the liner; aligning a fluid port of sleeve with a fluid port of packer mandrel extension, allowing wellbore pressure to enter a fluid pathway and act on the lower surface of piston; breaking the frangible member coupling the piston and the packer mandrel; and responsive to the pressure difference, longitudinally shifting the piston relative to the packer mandrel toward the seal assembly (100,102,104), to operate the seal assembly (100,102,104) from a running position to a radially expanded sealing position, thereby setting the packer. decoupling a collet assembly disposed about the packer mandrel from a sleeve slidably disposed about the packer mandrel with a liner in the wellbore; shifting the sleeve relative to a packer mandrel extension toward the piston by the movement of the packer causing the sleeve to contact a liner top of the liner; aligning a fluid port of sleeve with a fluid port of packer mandrel extension, allowing wellbore pressure to enter a fluid pathway and act on the lower surface of piston; breaking the frangible member coupling the piston and the packer mandrel; and responsive to the pressure difference, longitudinally shifting the piston relative to the packer mandrel toward the seal assembly (100,102,104), to operate the seal assembly (100,102,104) from a running position to a radially expanded sealing position, thereby setting the packer.