A method of mounting a subsea control module on a control module mounting structure comprising part of an underwater tree of a subsea hydrocarbon well, the method comprising: removing a prior deployed subsea control module, SCM, directly coupled to the control module mounting structure, the control module mounting structure forming part of an underwater tree of a subsea hydrocarbon well and comprising a mounting plate having a plurality of connectors mounted thereto, the prior deployed SCM having a base, a plurality of connectors mounted to the base according to a first arrangement to interface with the plurality of connectors mounted to the mounting plate of the control module mounting structure, and a locking shaft extending from the base and configured to extend into an orifice in the mounting plate of the control module mounting structure; lowering a replacement SCM to said control module mounting structure to replace the prior deployed SCM, the replacement SCM having a base and a plurality of connectors mounted to the base according to a second arrangement, the second arrangement of the plurality of connectors of the replacement SCM being different than the first arrangement of the plurality of connectors of the existing SCM, rendering the replacement SCM incompatible with direct coupling with the control module mounting structure; and attaching the replacement SCM to said control module mounting structure with an adapter between the replacement SCM and the control module mounting structure, the adapter providing for utilisation of the replacement SCM to provide for electrical and hydraulic control of the underwater tree, the adapter having a first mounting portion having a plurality of connectors extending therefrom arranged to interface with the plurality of connectors of the replacement SCM, and a second mounting portion having a plurality of connectors extending therefrom arranged to interface with the plurality of connectors mounted to the mounting plate of the control module mounting structure to thereby electrically and hydraulically connect the otherwise incompatible replacement SCM module with the underwater tree via the control module mounting structure. removing a prior deployed subsea control module, SCM, directly coupled to the control module mounting structure, the control module mounting structure forming part of an underwater tree of a subsea hydrocarbon well and comprising a mounting plate having a plurality of connectors mounted thereto, the prior deployed SCM having a base, a plurality of connectors mounted to the base according to a first arrangement to interface with the plurality of connectors mounted to the mounting plate of the control module mounting structure, and a locking shaft extending from the base and configured to extend into an orifice in the mounting plate of the control module mounting structure; lowering a replacement SCM to said control module mounting structure to replace the prior deployed SCM, the replacement SCM having a base and a plurality of connectors mounted to the base according to a second arrangement, the second arrangement of the plurality of connectors of the replacement SCM being different than the first arrangement of the plurality of connectors of the existing SCM, rendering the replacement SCM incompatible with direct coupling with the control module mounting structure; and attaching the replacement SCM to said control module mounting structure with an adapter between the replacement SCM and the control module mounting structure, the adapter providing for utilisation of the replacement SCM to provide for electrical and hydraulic control of the underwater tree, the adapter having a first mounting portion having a plurality of connectors extending therefrom arranged to interface with the plurality of connectors of the replacement SCM, and a second mounting portion having a plurality of connectors extending therefrom arranged to interface with the plurality of connectors mounted to the mounting plate of the control module mounting structure to thereby electrically and hydraulically connect the otherwise incompatible replacement SCM module with the underwater tree via the control module mounting structure.