A tool (1) for cementing an annulus in a subsea oil or gas well, and methods for using the tool are provided. The tool includes a safety module (2) providing fluid communication between an umbilical and a perforation and circulation module (4) mounted below it. The safety module (2) includes a mechanical lock for connection to a well head. The perforation and circulation module includes upper and lower seals (6, 8) for sealing to the inner surface of a casing; an upper perforating device (14) mounted between the seals; a lower perforating device (16) mounted below the lower seal (8); and supply (S) and return (R) fluid flow paths for circulating fluid from the safety module (2). A diversion means (20) is provided 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 (6, 8).