1. A method of operating a control system for controlling the manufacture of printed circuit boards, the steps performed by the control system comprising: modelling a production process in which a composite layer is formed from constituent layers, the modelling generating non-linear compensation data to compensate for non-linear distortion occurring in a circuit layout applied to at least one of the constituent layers and the modelling using stored values of parameters representing properties of respective materials of the constituent layers; outputting the compensation data for modifying a desired layout to generate a compensated layout to be applied in the production process; receiving measurement data representative of the actual circuit layout output from at least one phase of the production process utilizing the compensated layout; processing the measurement data to obtain optimized values of the parameters used in modelling; and updating the stored values of parameters with optimized values of the parameters used in modelling to be used in future modelling of the production process.