  A previous narrow-slit ($0.1$ arcsec) Hubble Space Telescope observation
unveiled a broad relativistic H$\alpha$ profile in NGC3147, a low-luminosity
($\mathrm{L_{bol}}\sim10^{42}$ erg s$^{-1}$), low-Eddington ratio
($\mathrm{L_{bol}/L_{Edd}}\sim10^{-4}$) active galactic nucleus (AGN), formerly
believed to be a candidate true type 2 AGN intrinsically lacking the broad-line
region. The new observations presented here confirm the double-peaked profile
of the H$\alpha$ line, which further shows variability both in flux and in the
inner radius of the emitting disc with respect to the previous epoch. Similar
disc line profiles are also found in prominent ultraviolet (UV) lines, in
particular Ly$\alpha$ and C IV. The new data also allow us to build a
simultaneous subarcsec optical-to-X-ray spectral energy distribution of
NGC3147, which is characterized by the absence of a thermal UV bump, and an
emission peak in the X-rays. The resulting very flat $\alpha_{ox}=-0.82$ is
typical of low-luminosity AGN, and is in good agreement with the extrapolation
to low luminosities of the well-known trend with luminosity observed in
standard AGN. Indeed, we are possibly observing the accretion disc emission in
NGC3147 in the optical, close to the expected peak. On the other hand, the
steep -2 UV power law may be Comptonization of that cold disc by a warm corona,
what is instead generally observed as a `soft excess' in more luminous AGN.
