A method of monitoring the production of hydrocarbons from a reservoir via a well comprising a dual completion, each completion producing from a different reservoir formation, said method comprising for each completion: obtaining first data describing a first relationship (210, 500) describing the variation of production flow rate with pressure between a point within the relevant reservoir formation and the well bottom; performing the following steps for each completion: obtaining plural sets of second data, each set of second data describing a second relationship (220, 510) describing the variation of production flow rate with pressure between the well bottom and the wellhead for one of a plurality of nominal values for a gas lift gas flow rate, said gas lift gas flow rate relating to the amount of gas lift gas introduced during production for the completion under consideration; characterised in using said first data and said second data to determine a third relationship describing the variation of a casing head pressure within the well with said gas lift gas flow rate, by determining each intersection of each set of second data with the first data, to obtain an intersection value (230, 520) for pressure and an intersection value (230, 520) for production flow rate for each of said sets of second data, said intersection values for pressure and production flow rate being used to determine the third relationship; using the determined third relationship and the assumption that the casing head pressure is the same for each completion to determine said gas lift gas flow rate for the completion under consideration. obtaining first data describing a first relationship (210, 500) describing the variation of production flow rate with pressure between a point within the relevant reservoir formation and the well bottom; performing the following steps for each completion: obtaining plural sets of second data, each set of second data describing a second relationship (220, 510) describing the variation of production flow rate with pressure between the well bottom and the wellhead for one of a plurality of nominal values for a gas lift gas flow rate, said gas lift gas flow rate relating to the amount of gas lift gas introduced during production for the completion under consideration; characterised in using said first data and said second data to determine a third relationship describing the variation of a casing head pressure within the well with said gas lift gas flow rate, by determining each intersection of each set of second data with the first data, to obtain an intersection value (230, 520) for pressure and an intersection value (230, 520) for production flow rate for each of said sets of second data, said intersection values for pressure and production flow rate being used to determine the third relationship; using the determined third relationship and the assumption that the casing head pressure is the same for each completion to determine said gas lift gas flow rate for the completion under consideration. obtaining plural sets of second data, each set of second data describing a second relationship (220, 510) describing the variation of production flow rate with pressure between the well bottom and the wellhead for one of a plurality of nominal values for a gas lift gas flow rate, said gas lift gas flow rate relating to the amount of gas lift gas introduced during production for the completion under consideration; characterised in using said first data and said second data to determine a third relationship describing the variation of a casing head pressure within the well with said gas lift gas flow rate, by determining each intersection of each set of second data with the first data, to obtain an intersection value (230, 520) for pressure and an intersection value (230, 520) for production flow rate for each of said sets of second data, said intersection values for pressure and production flow rate being used to determine the third relationship; using the determined third relationship and the assumption that the casing head pressure is the same for each completion to determine said gas lift gas flow rate for the completion under consideration.