I've upgraded the kernel to:
Linux duncan-e550 4.2.1-040201-generic #201509211431 SMP Mon Sep 21 18:34:44 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
.... which fixed a few other issues with the machine like suspend / resume behaviour, but the intermittent modem detection persists.
I've upgraded the kernel to:
Linux duncan-e550 4.2.1-040201- generic #201509211431 SMP Mon Sep 21 18:34:44 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
.... which fixed a few other issues with the machine like suspend / resume behaviour, but the intermittent modem detection persists.