Both KH & WL have catacombs. The idea came from the Med area - Victorians coming back from the Grand Tour!
Although more expensive to build a brick 'storeroom' initially, it was cheaper in the long run for burials. You could inter 12 persons by just unlocking a glass dorr and popping the on the shelf! Had you purchased a family grave - you would have had to dig it out and fill it up again 12 times - a lot more manpower!
Although envisaged for families, the concept never really took off, although some were purchased privately. The majority were used for Public Interments - Ordinary working class people, who could not afford to purchase outrright a complete grave, so 'rented' or 'timeshared' a space with others.
Most Public catacombs contain 12 non related persons, although being down to earth brummies, I'm sure they have all got their back doors unlocked and get on fine - just as they did in life!
Shame your father in law never knew his roots - so close and yet so far.......