  We present milliarcsecond (mas) angular resolution observations of the
gravitationally lensed radio source MG J0751+2716 (at z=3.2) obtained with
global Very Long Baseline Interferometry (VLBI) at 1.65 GHz. The background
object is highly resolved in the tangential and radial directions, showing
evidence of both compact and extended structure across several gravitational
arcs that are 200 to 600~mas in size. By identifying compact sub-components in
the multiple images, we constrain the mass distribution of the foreground
z=0.35 gravitational lens using analytic models for the main deflector
[power-law elliptical mass model; $\rho(r) \propto r^{-\gamma}$, where
$\gamma=2$ corresponds to isothermal] and for the members of the galaxy group.
Moreover, our mass models with and without the group find an inner mass-density
slope steeper than isothermal for the main lensing galaxy, with $\gamma_1 =
2.08 \pm 0.02$ and $\gamma_2 = 2.16 \pm 0.02$ at the 4.2$\sigma$ level and
6.8$\sigma$ level, respectively, at the Einstein radius ($b_1 = 0.4025 \pm
0.0008$ and $b_2 = 0.307 \pm 0.002$ arcsec, respectively). We find randomly
distributed image position residuals of about 3 mas, which are much larger that
the measurement errors ($40$ $\mu$as on average). This suggests that at the mas
level, the assumption of a smooth mass distribution fails, requiring additional
structure in the model. However, given the environment of the lensing galaxy,
it is not clear whether this extra mass is in the form of sub-haloes within the
lens or along the line of sight, or from a more complex halo for the galaxy
group.
