Lath martensite reveals a specific hierarchical subgrain structure, with laths, blocks and packets of particular crystallography. The presence of interlath retained austenite layers has been reported in the literature. This paper investigates the potential influence of the interlath retained austenite on the mechanical behaviour of lath martensite subgrains. To this purpose, a martensite grain substructure is modelled using a crystal plasticity framework, with a BCC lath - FCC austenite bicrystal at the fine scale. The main novel contribution of this work is the validation of the hypothesis on the role of the interlath retained austenite in lath martensite using the experimental results reported in the literature. The main features of the experimentally observed deformation behaviour (stress-strain curve, slip activity and roughness pattern) are qualitatively well reproduced by the model. It is shown that the presence of austenite interlath films has the potential to remarkably enhance the local deformation of martensite. In spite of its minor volume fraction, it plays a major role in the orientation dependent mechanical behaviour of the aggregate. It is also shown that if the presence of interlath austenite is neglected, the observed experimental flow curves are not captured.