A method of accessing an oil or gas containing formation by forming a window in the casing of a cased wellbore to permit sidetrack entry to the formation, the method being characterised by comprising: deploying a tool assembly upon a work string via a blow out preventer stack (B.O.P.) and through a wellhead into the casing, said tool assembly comprising a milling and / or drilling tool adapted to penetrate the casing and a B.O.P. protection device comprising a releasable sleeve assembly (2, 3) and a running tool (1, 4, 5) for same, wherein the running tool comprises a running tool housing connected to the releasable sleeve assembly (2, 3) and a separable running tool body connected into the work string within its length, the running tool housing being connected to the running tool body by releasable fasteners for running in hole, and wherein the running tool housing is configured to land on a surface within a subsea wellhead structure; landing the running tool (1, 4, 5) to effect release of the fasteners and separation of the running tool housing from running tool body to deploy the releasable sleeve assembly (2, 3) within the B.O.P., wherein the B.O.P. protection device is dimensioned and positioned to locate the releasable sleeve assembly (2, 3) between lower annular seal and rams (20, 21, 22, 23) forming contingency measures of the B.O.P, wherein a sleeve of said releasable sleeve assembly (2, 3) is configured to define a fluid circulation path around the work string within the B.O.P., and form a barrier to ingress of deleterious materials into said contingency measures of the B.O.P.; running the milling and / or drilling tool on downhole within the casing; and milling a window in the casing and optionally drilling into the formation. deploying a tool assembly upon a work string via a blow out preventer stack (B.O.P.) and through a wellhead into the casing, said tool assembly comprising a milling and / or drilling tool adapted to penetrate the casing and a B.O.P. protection device comprising a releasable sleeve assembly (2, 3) and a running tool (1, 4, 5) for same, wherein the running tool comprises a running tool housing connected to the releasable sleeve assembly (2, 3) and a separable running tool body connected into the work string within its length, the running tool housing being connected to the running tool body by releasable fasteners for running in hole, and wherein the running tool housing is configured to land on a surface within a subsea wellhead structure; landing the running tool (1, 4, 5) to effect release of the fasteners and separation of the running tool housing from running tool body to deploy the releasable sleeve assembly (2, 3) within the B.O.P., wherein the B.O.P. protection device is dimensioned and positioned to locate the releasable sleeve assembly (2, 3) between lower annular seal and rams (20, 21, 22, 23) forming contingency measures of the B.O.P, wherein a sleeve of said releasable sleeve assembly (2, 3) is configured to define a fluid circulation path around the work string within the B.O.P., and form a barrier to ingress of deleterious materials into said contingency measures of the B.O.P.; running the milling and / or drilling tool on downhole within the casing; and milling a window in the casing and optionally drilling into the formation.