  We present an analysis of observations of the doubly-lensed double hotspot in
the giant radio galaxy J0816+5003 from MERLIN, MDM, WIYN, WHT, UKIRT and the
VLA. The images of the two hotspot components span a factor of two in radius on
one side of the lensing galaxy at impact parameters of less than 500pc. Hence
we measure the slope of the lensing potential over a large range in radius,
made possible by significant improvement in the accuracy of registration of the
radio and optical frame and higher resolution imaging data than previously
available. We also infer the lens and source redshifts to be 0.332 and > 1
respectively. Purely on the basis of lens modelling, and independently of
stellar velocity dispersion measurements, we find the potential to be very
close to isothermal.
