  We have observed the relativistic binary pulsar PSR J1141$-$6545 over a
period of $\sim$6 years using the Parkes 64 m radio telescope, with a focus on
modelling the diffractive intensity scintillations to improve the accuracy of
the astrometric timing model. The long-term scintillation, which shows orbital
and annual variations, allows us to measure parameters that are difficult to
measure with pulsar timing alone. These include: the orbital inclination $i$;
the longitude of the ascending node $\Omega$; and the pulsar system transverse
velocity. We use the annual variations to resolve the previous ambiguity in the
sense of the inclination angle. Using the correct sense, and a prior
probability distribution given by a constraint from pulsar timing
($i=73\pm3^\circ$), we find $\Omega=24.8\pm1.8^\circ$ and we estimate the
pulsar distance to be $D=10^{+4}_{-3}$ kpc. This then gives us an estimate of
this pulsar's proper motion of $\mu_{\alpha}\cos{\delta}=2.9\pm1.0$ mas
yr$^{-1}$ in right ascension and $\mu_{\delta}=1.8\pm0.6$ mas yr$^{-1}$ in
declination. Finally, we obtain measurements of the spatial structure of the
interstellar electron density fluctuations, including: the spatial scale and
anisotropy of the diffraction pattern; the distribution of scattering material
along the line of sight; and spatial variation in the strength of turbulence
from epoch to epoch. We find that the scattering is dominated by a thin screen
at a distance of $(0.724\pm0.008)D$, with an anisotropy axial ratio $A_{\rm r}
= 2.14\pm0.11$.
