  To advance our understanding of the fuelling and feedback processes which
power the Universe's most massive black holes, we require a significant
increase in our knowledge of the molecular gas which exists in their immediate
surroundings. However, the behaviour of this gas is poorly understood due to
the difficulties associated with observing it directly. We report on a survey
of 18 brightest cluster galaxies lying in cool cores, from which we detect
molecular gas in the core regions of eight via carbon monoxide (CO), cyanide
(CN) and silicon monoxide (SiO) absorption lines. These absorption lines are
produced by cold molecular gas clouds which lie along the line of sight to the
bright continuum sources at the galaxy centres. As such, they can be used to
determine many properties of the molecular gas which may go on to fuel
supermassive black hole accretion and AGN feedback mechanisms. The absorption
regions detected have velocities ranging from -45 to 283 km s$^{-1}$ relative
to the systemic velocity of the galaxy, and have a bias for motion towards the
host supermassive black hole. We find that the CN N = 0 - 1 absorption lines
are typically 10 times stronger than those of CO J = 0 - 1. This is due to the
higher electric dipole moment of the CN molecule, which enhances its absorption
strength. In terms of molecular number density CO remains the more prevalent
molecule with a ratio of CO/CN $\sim 10$, similar to that of nearby galaxies.
Comparison of CO, CN and HI observations for these systems shows many different
combinations of these absorption lines being detected.
