A riser-less method of performing a plug and abandonment operation on a subsea hydrocarbon well from a vessel, the method comprising: providing an apparatus, the apparatus comprising: a wellhead interface module located on a wellhead, the wellhead interface module comprising a body defining a chamber which accommodates a volume of wellbore fluid in fluid communication with the wellbore, an upper end of the chamber being open to the subsea environment; a subsea flow control package; a fluid conduit extending between the subsea flow control package and surface; and a system control module; wherein the subsea flow control package defines a first flow path between wellhead interface module and the fluid conduit via a pump and defines a second flow path between the wellhead interface module and the fluid conduit via a flow control valve; removing a length of tubing or casing from the wellbore; controlling, using the subsea flow control package, the flow of a wellbore fluid from the fluid conduit to re-fill the chamber of the wellhead interface module; enabling wellbore fluid to flow from the chamber into the wellbore; measuring, using a level sensor of the wellhead interface module, a level of the wellbore fluid in the chamber and outputting a measurement signal to the system control module in dependence on the level of wellbore fluid; deriving volume data relating to a change in volume of wellbore fluid in the chamber, said change in volume being derived from the change in the level of the wellbore fluid in the chamber; and comparing the derived volume data with a volume change expected due to the removal of tubing or casing from the wellbore. providing an apparatus, the apparatus comprising: a wellhead interface module located on a wellhead, the wellhead interface module comprising a body defining a chamber which accommodates a volume of wellbore fluid in fluid communication with the wellbore, an upper end of the chamber being open to the subsea environment; a subsea flow control package; a fluid conduit extending between the subsea flow control package and surface; and a system control module; wherein the subsea flow control package defines a first flow path between wellhead interface module and the fluid conduit via a pump and defines a second flow path between the wellhead interface module and the fluid conduit via a flow control valve; a wellhead interface module located on a wellhead, the wellhead interface module comprising a body defining a chamber which accommodates a volume of wellbore fluid in fluid communication with the wellbore, an upper end of the chamber being open to the subsea environment; a subsea flow control package; a fluid conduit extending between the subsea flow control package and surface; and a system control module; wherein the subsea flow control package defines a first flow path between wellhead interface module and the fluid conduit via a pump and defines a second flow path between the wellhead interface module and the fluid conduit via a flow control valve; removing a length of tubing or casing from the wellbore; controlling, using the subsea flow control package, the flow of a wellbore fluid from the fluid conduit to re-fill the chamber of the wellhead interface module; enabling wellbore fluid to flow from the chamber into the wellbore; measuring, using a level sensor of the wellhead interface module, a level of the wellbore fluid in the chamber and outputting a measurement signal to the system control module in dependence on the level of wellbore fluid; deriving volume data relating to a change in volume of wellbore fluid in the chamber, said change in volume being derived from the change in the level of the wellbore fluid in the chamber; and comparing the derived volume data with a volume change expected due to the removal of tubing or casing from the wellbore.