@Graham Clinch,
A potential workaround could be to install libgl1-mesa-glx package for the missing bits.
$ apt-file search libGL.so.1 ... libgl1-mesa-glx: /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1 libgl1-mesa-glx: /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1.2.0 ...
@Graham Clinch,
A potential workaround could be to install libgl1-mesa-glx package for the missing bits.
$ apt-file search libGL.so.1 x86_64- linux-gnu/ mesa/libGL. so.1 x86_64- linux-gnu/ mesa/libGL. so.1.2. 0
...
libgl1-mesa-glx: /usr/lib/
libgl1-mesa-glx: /usr/lib/
...