  HIP 65426 b is a recently discovered exoplanet imaged during the course of
the SPHERE-SHINE survey. Here we present new $L'$ and $M'$ observations of the
planet from the NACO instrument at the VLT from the NACO-ISPY survey, as well
as a new $Y-H$ spectrum and $K$-band photometry from SPHERE-SHINE. Using these
data, we confirm the nature of the companion as a warm, dusty planet with a
mid-L spectral type. From comparison of its SED with the BT-Settl atmospheric
models, we derive a best-fit effective temperature of $T_{\text{eff}}=1618\pm7$
K, surface gravity $\log g=3.78^{+0.04}_{-0.03}$ and radius $R=1.17\pm0.04$
$R_{\text{J}}$ (statistical uncertainties only). Using the DUSTY and COND
isochrones we estimate a mass of $8\pm1$ $M_{\text{J}}$. Combining the
astrometric measurements from our new datasets and from the literature, we show
the first indications of orbital motion of the companion (2.6$\sigma$
significance) and derive preliminary orbital constraints. We find a highly
inclined orbit ($i=107^{+13}_{-10}$ deg) with an orbital period of
$800^{+1200}_{-400}$ yr. We also report SPHERE sparse aperture masking
observations that investigate the possibility that HIP 65426 b was scattered
onto its current orbit by an additional companion at a smaller orbital
separation. From this data we rule out the presence of brown dwarf companions
with masses greater than 16 $M_{\text{J}}$ at separations larger than 3 AU,
significantly narrowing the parameter space for such a companion.
