  In order to shed light on the characteristics of the broad line region (BLR)
in a narrow-line Seyfert 1 galaxy, we present an analysis of X-ray, UV, and
optical spectroscopic observations of the broad emission lines in Mrk 110. For
the broad-band modelling of the emission-line luminosity, we adopt the `locally
optimally emitting cloud' approach, which allows us to place constraints on the
gas radial and density distribution. By exploring additional environmental
effects, we investigate the possible scenarios resulting in the observed
spectra. We find that the photoionised gas in Mrk 110 responsible for the UV
emission can fully account for the observed low-ionisation X-ray lines. The
overall ionisation of the gas is lower, and one radial power-law distribution
with a high integrated covering fraction $C_{\mathrm{f}} \approx 0.5$ provides
an acceptable description of the emission lines spanning from X-rays to the
optical band. The BLR is likely more compact than the broad-line Seyfert 1s
studied so far, extending from $\sim\!10^{16}$ to $\sim\!10^{18}$ cm, and
limited by the dust sublimation radius at the outer edge. Despite the large
colour excess predicted by the Balmer ratio, the best fit suggests
$E(B-V)\approx0.03$ for both the ionising luminosity and the BLR, indicating
that extinction might be uniform over a range of viewing angles. While the
adopted data-modelling technique does not allow us to place constraints on the
geometry of the BLR, we show that the addition of models with a clumpy,
equatorial, wind-like structure may lead to a better description of the
observed spectra.
