Appararus and method for inserting and retrieving a tool string through well surface equipment

A method of deploying a tool string 6 comprising inserting a first tool into a wellbore 11 through well surface equipment 50, isolating a first portion of the well surface equipment from the elevated wellbore pressure, possibly by isolating an inner chamber of a lubricator (54, figure 1) using engaging rams (63, figure 5). Connecting a second tool to the first tool in the portion of the well surface equipment isolated from the pressure, the first tool and second tool forming part of the tool string. The tool string having an inner bore that is opened to fluid communication in response to activation of the tool string (e.g ballistically via a detonation cord) and providing a barrier mechanism to block one portion of the inner bore from another portion after activation of the tool string. The barrier mechanism is actuated using a pressure activated mechanism and may be a valve e.g a flapper valve (600, figure 10A), a plug (410, figure 9) or a through-bulk initiator assembly (330, figure 6).