Method of controlling rail pressure

A method for controlling rail pressure in a fuel supply system comprising a fuel pump 1, at least one injector 4 and a rail 3 connecting the injector to the pump, comprises the steps of establishing a relationship between rail pressure and a leak rate of the injector, estimating a fuel drain rate from the rail based on a fuel injection rate, rail pressure and rail pressure/leak rate relationship; estimating a desired intake flow rate of the pump based on fuel drain rate and controlling the pump to operate at a desired intake flow rate. The method may also comprise the step of establishing a relationship between rail pressure and efficiency of the pump to estimate the desired intake flow and Preferably the rail pressure/leak rate relationship is established as a function of injector excitation time or fuel temperature, specifies an engine speed weighted sum of at least a static leak rate and a dynamic leak rate. A controller is also claimed.