So, Running a combination of testing and unstable on my Bytemark UML instance, and over the course of time, the NTPL libraries started to turn up in /lib/tls. Turns out that UML and NTPL are incompatible: every binary I used that utilised threads, such as MySQL, Bind9, Nslookup, all segfault when these libraries are accesible.
Googling around shows that NTPL support doesnt work under UML: the work around is to rename the /lib/tls directory (eg, /lib/tls.disable) so that it can’t be found.