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Summary form only given. We have developed an inter-/intra-band double-resonance technique that has allowed us to make an unequivocal assignment of the resonant bound-bound intersublevel absorption in self-organized InAs/GaAs quantum dots. We have applied the technique to dots grown at a low growth-rate, which show well resolved photoluminescence (PL) lines. The double resonance (far infrared modulated photoluminescence, FIR-MPL) and its polarisation selection rules allows us to identify not only the levels involved in the PL transition but the relative importance of the causes of its inhomogeneous broadening. The PL and the FIR-MPL were collected simultaneously to ensure the correspondence of features in each by monitoring both the DC and AC outputs of the detector. Spectra were obtained by scanning either the monochrometer at fixed FIR frequency or scanning FELIX at a fixed PL frequency. The results unequivocally confirm that the interband PL lines arise from successively increasing in-plane quantum numbers, with the lowest electronic states separated by 55 meV and corresponding hole states by 13 meV. Most importantly we believe that our method can distinguish between the three PL inhomogeneous broadening mechanisms expected to be significant in our self-assembled system of dots (a) in-plane dot size variation, (b) out-of-plane or potential broadening, and (c) compositional broadening associated with fluctuations of In within the dot.