A method of creating an annular zonal isolation seal in a downhole annulus between a string of downhole tubular joints inserted within a bore in borehole in the Earth, and an inner wall of the bore, comprising: - including an isolation joint in the string, said isolation joint comprising a downhole tubular of a predetermined length in an axial direction, comprising a tube wall having a wall thickness that varies in the axial direction over said length, whereby at least one expandable section is provided in the downhole tubular which, in axial direction, is sandwiched between a first separator section and a second separator section of the downhole tubular, wherein said at least one expandable section has a circumferential band of increased wall thickness compared to the wall thicknesses of the first and second separator sections, and said downhole tubular further providing a mating support at a predetermined axial location relative to said at least expandable section, adapted for mating with a local expander device within said downhole tubular; - providing a local expander device comprising: - a force imparting section configured to impart a transversely directed expansion force to an expandable section of the downhole tubular, and - a carrier section, carrying the force imparting section, and comprising a complementary mating support at a predetermined axial separation distance relative to the force imparting section; - lowering said local expander device through the downhole tubular string and at least partially into the isolation joint; - mating the local expander device in position by engaging the complementary mating support with said mating support of the downhole tubular whereby the force imparting section of the local expander device is in transverse alignment with said circumferential band of increased wall thickness in the expandable section of the downhole tubular; - activating the force imparting section of the local expander device, thereby imparting the transversely directed expansion force to the expandable section of the downhole tubular whereby locally expanding the downhole tubular into the downhole annulus; characterized in that the transversely directed expansion force imparts a radial strain on the isolation joint exclusively within the or each circumferential band of increased wall thickness. - including an isolation joint in the string, said isolation joint comprising a downhole tubular of a predetermined length in an axial direction, comprising a tube wall having a wall thickness that varies in the axial direction over said length, whereby at least one expandable section is provided in the downhole tubular which, in axial direction, is sandwiched between a first separator section and a second separator section of the downhole tubular, wherein said at least one expandable section has a circumferential band of increased wall thickness compared to the wall thicknesses of the first and second separator sections, and said downhole tubular further providing a mating support at a predetermined axial location relative to said at least expandable section, adapted for mating with a local expander device within said downhole tubular; - providing a local expander device comprising: - a force imparting section configured to impart a transversely directed expansion force to an expandable section of the downhole tubular, and - a carrier section, carrying the force imparting section, and comprising a complementary mating support at a predetermined axial separation distance relative to the force imparting section; - lowering said local expander device through the downhole tubular string and at least partially into the isolation joint; - mating the local expander device in position by engaging the complementary mating support with said mating support of the downhole tubular whereby the force imparting section of the local expander device is in transverse alignment with said circumferential band of increased wall thickness in the expandable section of the downhole tubular; - activating the force imparting section of the local expander device, thereby imparting the transversely directed expansion force to the expandable section of the downhole tubular whereby locally expanding the downhole tubular into the downhole annulus; - a force imparting section configured to impart a transversely directed expansion force to an expandable section of the downhole tubular, and - a carrier section, carrying the force imparting section, and comprising a complementary mating support at a predetermined axial separation distance relative to the force imparting section; - lowering said local expander device through the downhole tubular string and at least partially into the isolation joint; - mating the local expander device in position by engaging the complementary mating support with said mating support of the downhole tubular whereby the force imparting section of the local expander device is in transverse alignment with said circumferential band of increased wall thickness in the expandable section of the downhole tubular; - activating the force imparting section of the local expander device, thereby imparting the transversely directed expansion force to the expandable section of the downhole tubular whereby locally expanding the downhole tubular into the downhole annulus;