A method of performing zonal isolation during a frac operation with a casing/liner string (171, 203, 205) that has been pre-perforated, the method comprising the steps of:- a) drilling the borehole (163, 213), b) run in completion which may be in the form of a casing/liner string and which is to be permanently installed in the open hole borehole, wherein at least one zonal isolation device is provided on or associated with the casing/liner string, the zonal isolation device comprising a sleeve member (43a, 43b, 173) defining a chamber into which pressurised fluid can be inserted, through an aperture in the casing/liner string that is surrounded by the sleeve member, to permanently expand the sleeve member outwards towards the open hole borehole; c) run a tool (140,190) into the throughbore (17, 29, 223) of the casing/liner string into the vicinity of the pre-perforated liner and operate the tool to introduce fluid under pressure into the throughbore of the casing/liner string section to expand and thereby activate the zonal isolation device(s) such that the at least one zonal isolation device provides a seal against the open hole, and repeat step c) for any other required zonal isolation device(s) and once step c) is completed, withdraw tool of step c) from the borehole; d) close the inner bore of the casing/liner string at some point vertically below the lower most zonal isolation device; e) supply frac fluid through the perforations in the casing/liner string to the zone requiring to be frac'd in order to perform the frac; and f) repeat steps c), d) and e) as required for each additional zone to be frac'd; whereby the pressure of the frac fluid supplied in step e) acts not only on the outside of the zonal isolation device but also on the interior of the zonal isolation device, directly from the throughbore of the casing/liner string via the same said aperture, to enhance the seal provided thereby. a) drilling the borehole (163, 213), b) run in completion which may be in the form of a casing/liner string and which is to be permanently installed in the open hole borehole, wherein at least one zonal isolation device is provided on or associated with the casing/liner string, the zonal isolation device comprising a sleeve member (43a, 43b, 173) defining a chamber into which pressurised fluid can be inserted, through an aperture in the casing/liner string that is surrounded by the sleeve member, to permanently expand the sleeve member outwards towards the open hole borehole; c) run a tool (140,190) into the throughbore (17, 29, 223) of the casing/liner string into the vicinity of the pre-perforated liner and operate the tool to introduce fluid under pressure into the throughbore of the casing/liner string section to expand and thereby activate the zonal isolation device(s) such that the at least one zonal isolation device provides a seal against the open hole, and repeat step c) for any other required zonal isolation device(s) and once step c) is completed, withdraw tool of step c) from the borehole; d) close the inner bore of the casing/liner string at some point vertically below the lower most zonal isolation device; e) supply frac fluid through the perforations in the casing/liner string to the zone requiring to be frac'd in order to perform the frac; and f) repeat steps c), d) and e) as required for each additional zone to be frac'd; whereby the pressure of the frac fluid supplied in step e) acts not only on the outside of the zonal isolation device but also on the interior of the zonal isolation device, directly from the throughbore of the casing/liner string via the same said aperture, to enhance the seal provided thereby.