  We present the results of our recent Hubble Space Telescope imaging study, in
which we have successfully deconvolved host and nuclear flux for the some of
the most luminous quasars in the Universe. Host morphologies have been
recovered for each of our 17-strong sample. From these fits, we have estimated
Black Hole masses through extrapolation of the Black Hole - Spheroid mass
relation and begun to address the complicated issue of fuelling vs black hole
mass in determining quasar luminosity: We find that the order-of-magnitude
increase in luminosity is due to increased black hole size, and to increased
fuelling efficiency, in roughly equal measure. The brightest objects are found
to radiate at or near the Eddington limit.
