An overshot tool for releasable connection to a head assembly of a core barrel drilling apparatus, the tool comprising: a primary engaging portion to engage a latch of the head assembly to provide an axial couple between the tool and the head assembly; a secondary engaging portion to temporarily engage the latch in addition to the engagement of the latch by the primary engaging portion; a retainer comprising a shaft and a head, which retainer acts between the primary engaging portion and the secondary engaging portion; a housing having a region (29, 60) for engagement by the retainer, the primary engaging portion and the secondary engaging portion axially movable relative to the housing; the retainer configured i) to engage a first part of the region (29, 60) to releasably couple the primary and secondary engaging portions for combined axial movement and ii) to engage a second part of the region (29, 60) to allow at least partial independent axial movement of the secondary engaging portion relative to the primary engaging portion, wherein by adjustment of a position of the retainer between the first part and the second part of the housing the tool is adjustable between a first mode to allow axial coupling between the tool and the head assembly and a second mode to provide a decoupling of the tool from the head assembly; a bias member acting between the housing and the secondary engaging portion to bias the secondary engaging portion axially relative to the primary engaging portion; characterised in that the primary engaging portion comprises an elongate shaft and the secondary engaging portion comprises a sleeve positioned around the shaft, the sleeve configured to slide axially over the shaft. a primary engaging portion to engage a latch of the head assembly to provide an axial couple between the tool and the head assembly; a secondary engaging portion to temporarily engage the latch in addition to the engagement of the latch by the primary engaging portion; a retainer comprising a shaft and a head, which retainer acts between the primary engaging portion and the secondary engaging portion; a housing having a region (29, 60) for engagement by the retainer, the primary engaging portion and the secondary engaging portion axially movable relative to the housing; the retainer configured i) to engage a first part of the region (29, 60) to releasably couple the primary and secondary engaging portions for combined axial movement and ii) to engage a second part of the region (29, 60) to allow at least partial independent axial movement of the secondary engaging portion relative to the primary engaging portion, wherein by adjustment of a position of the retainer between the first part and the second part of the housing the tool is adjustable between a first mode to allow axial coupling between the tool and the head assembly and a second mode to provide a decoupling of the tool from the head assembly; a bias member acting between the housing and the secondary engaging portion to bias the secondary engaging portion axially relative to the primary engaging portion; characterised in that the primary engaging portion comprises an elongate shaft and the secondary engaging portion comprises a sleeve positioned around the shaft, the sleeve configured to slide axially over the shaft.