A tool for cementing an annulus in a subsea oil or gas well, the tool comprising: a safety module and a perforation and circulation module; wherein the safety module provides fluid communication between an umbilical and the perforation and circulation module, and includes a mechanical lock formed of moving parts that are movable outwards into contact with an inside surface of a well head and wherein the moving parts are arranged to move into locking engagement with the inside surface of the well head in use; and wherein the perforation and circulation module is mounted below the safety module, and comprises: i) upper and lower seals (6, 8) for sealing to the inner surface of a casing inside a wellbore; ii) at least one upper perforating device, mounted between the upper and lower seals (6, 8), for perforating casing; iii) at least one lower perforating device, mounted below the lower seal, for perforating wellbore casing; iv) a supply fluid flow path to supply fluid from the safety module through the upper and lower seals (6, 8) to below the perforation and circulation module; v) a return fluid flow path from between the upper and lower seals (6, 8) to the safety module; and vi) a diversion means in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals. a safety module and a perforation and circulation module; wherein the safety module provides fluid communication between an umbilical and the perforation and circulation module, and includes a mechanical lock formed of moving parts that are movable outwards into contact with an inside surface of a well head and wherein the moving parts are arranged to move into locking engagement with the inside surface of the well head in use; and wherein the perforation and circulation module is mounted below the safety module, and comprises: i) upper and lower seals (6, 8) for sealing to the inner surface of a casing inside a wellbore; ii) at least one upper perforating device, mounted between the upper and lower seals (6, 8), for perforating casing; iii) at least one lower perforating device, mounted below the lower seal, for perforating wellbore casing; iv) a supply fluid flow path to supply fluid from the safety module through the upper and lower seals (6, 8) to below the perforation and circulation module; v) a return fluid flow path from between the upper and lower seals (6, 8) to the safety module; and vi) a diversion means in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals. i) upper and lower seals (6, 8) for sealing to the inner surface of a casing inside a wellbore; ii) at least one upper perforating device, mounted between the upper and lower seals (6, 8), for perforating casing; iii) at least one lower perforating device, mounted below the lower seal, for perforating wellbore casing; iv) a supply fluid flow path to supply fluid from the safety module through the upper and lower seals (6, 8) to below the perforation and circulation module; v) a return fluid flow path from between the upper and lower seals (6, 8) to the safety module; and vi) a diversion means in the supply fluid flow path, operable to redirect fluid supplied to the supply fluid flow path to a space defined between the upper and lower seals.