  We present high-resolution mass reconstructions for five massive
cluster-lenses spanning a redshift range from $z = 0.18$--0.57 utilising
archival {\it Hubble Space Telescope} ({\it HST}) data and applying
galaxy-galaxy lensing techniques. These detailed mass models were obtained from
the observations by combining constraints from the strong and weak lensing
regimes. We ascribe local weak distortions in the shear maps to perturbations
induced by the presence of galaxy haloes around individual bright early-type
cluster member galaxies. This technique constrains the mass enclosed within an
aperture for these subhaloes. We are sensitive to a specific mass range for
these subhaloes, $10^{11}$ -- $10^{12.5} \msun$, which we associate with
galaxy-scale subhaloes. Adopting a parametric model for the subhaloes, we also
derive their velocity dispersion function and the aperture radius function. The
mass spectrum of substructure in the inner regions of the observed clusters is
directly compared with that in simulated clusters extracted from the {\it
Millennium Simulation}. The massfunction, aperture radii and velocity
dispersion function are compared in detail. Overall, we find good agreement
between the distribution of substructure properties retrieved using the lensing
analysis and those obtained from the simulation (truncated abstract).
