  We have determined the structure and kinematics of ionised gas in the weak
oval bar of the archetypal Seyfert 1 galaxy, NGC 4151, using the TAURUS
Fabry-Perot interferometer to simultaneously map the distribution and
kinematics of Hbeta emission. We also present broad-band ultraviolet imaging of
the host galaxy, obtained with XMM-Newton, that shows the detailed distribution
of star formation in the bar and in the optically-faint outer spiral arms. We
compare the distribution and kinematics of ionised gas with that previously
determined in neutral hydrogen by Mundell & Shone; we suggest that the
distribution of bright, patchy UV emission close to the HI shocks is consistent
with ionisation by star clusters that have formed in compressed pre-shock gas.
These clusters then travel ballistically through the gaseous shock to ionise
gas downstream along the leading edge of the bar. In addition, we detect, for
the first time, ionised gas within the shock itself which is streaming to
smaller radii in the same manner as the neutral gas.
