In the absence of 50 m class space-based observatories, sub-arc-second astronomy spanning the full far-infrared wavelength range will require space-based long-baseline interferometry. The long baselines of up to 10’s of meteres are necessary to achieve sub arcsecond resolution demanded by the science goals. Also, practical observing times command a field of view toward an arc minute or so, not achievable with a single on-axis coherent detector. This paper is concerned with an application of an end-to-end instrument simulator PyFIInS, developed as part of the FISICA project under funding from the European Commission’s 7th Framework Programme for Research and Technological Development (FP7). Predicted results of wide field of view spatio-spectral interferometry through simulations of a long-baseline, double-Fourier, far-infrared interferometer concept are presented and analysed. It is shown how such an interferometer, illuminated by a multimode detector can recover a large field of view at sub-arcsecond angular resolution, resulting in similar image quality as that achieved by illuminating the system with an array of coherent detectors. Through careful analysis, the importance of accounting for the correct number of higher-order optical modes is demonstrated, as well as accounting for both orthogonal polarisations. Given that it is very difficult to manufacture waveguide and feed structures at sub-mm wavelengths, the larger multimode design is recommended over the array of smaller single mode detectors. A brief note is provided in the conclusion of this paper, addressing a novel, more elegant solution to modelling far-infrared interferometers, which holds promise for improving the computational efficiency of the simulations presented here.