An original diagrammatic technique is advanced for calculating, in the first-harmonic approximation of the Weak Segregation Limit theory, the coefficients of the amplitude expansion of the Landau free energy for a melt of a binary incompressible heteropolymer comprising linear macromolecules with arbitrary chemical structure. A universal approach to the calculation of these coefficients for heteropolymer liquids describable using the Landau theory of phase transitions with a two-component order parameter is put forward for the first time. This approach proves to be particularly efficient in a theoretical consideration of the thermodynamic behaviour of multiblock copolymers whose macromolecules contain blocks of two types substantially differing in length. A general algorithm for finding the coefficients of the amplitude expansion of the Landau free energy in such systems is formulated for a number of mesophases showing two characteristic scales of spatial periodicity. The application of this algorithm is exemplified by considering a deformed hexagonally perforated lamellar mesophase.