We use 3D printing to manufacture lattices with uniform and graded relative density, made from a composite parent material comprising a nylon matrix reinforced by short carbon fibres. The elastic-plastic compressive response of these solids is measured up to their densification regime. Data from experiments on the lattices with uniform relative density is used to deduce the dependence of their elastic-plastic homogenised constitutive response on their relative density, in the range 0.2-0.8. This data is used to calibrate Finite Element (FE) simulations of the compressive response of Functionally Graded Lattices (FGLs), which are found in good agreement with the corresponding measurements, capturing the salient features of the measured stress versus strain responses. This exercise is repeated for two lattice topologies (body-centred cubic and Schwarz-P). The phenomenological constitutive models produced in this study can be used in topology optimisation to maximise the performance of 3D printed FGLs components in terms of stiffness, strength or energy absorption.