16.04 xenial upgrade hangs

I am upgrading one of my desktops (a run-of-the-mill 3-core AMD-64 system), from 15.10 to 16.04, xenial, and am seeing the update manager hanging at the "Installing the upgrades" stage, having successfully passed the "Preparing to upgrade", "Setting new software chaels", and the "Getting new packages" stages. The terminal is stuck at "Preparing to unpack .../rpcbind_0.2.3-0.2_amd64.deb ..." (part of the Unpacking wpasupplicant step I think).
I can see these processes ruing:
hai 26432 0.0 0.0 4476 700 ? S 09:48 0:06 /bin/sh -c /usr/bin/pkexec /usr/bin/do-release-upgrade --frontend=DistUpgradeViewGtk3
root 26494 1.5 3.9 611816 140464 ? Sl 09:48 4:52 /usr/bin/python3 /tmp/ubuntu-release-upgrader-jz0ku4aw/xenial --mode=server --frontend=DistUpgradeViewGtk3
root 27977 8.1 3.1 570388 112220 pts/1 Ss+ 11:26 17:16 /usr/bin/python3 /tmp/ubuntu-release-upgrader-jz0ku4aw/xenial --mode=server --frontend=DistUpgradeViewGtk3

Using strace I can see that the process "/usr/bin/pkexec" is just stuck on a wait4(), while the xenial python processes are screaming along indefinitely with:
...
select(0, NULL, NULL, NULL, {0, 10000}) = 0 (Timeout)
read(15, 0x2faca30, 8192) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(8, 0x7ffce2f78cd0, 0) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=5, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=62, events=POLLIN}], 4, 0) = 0 (Timeout)
select(0, NULL, NULL, NULL, {0, 10000}) = 0 (Timeout)
read(15, 0x2faca30, 8192) = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(8, 0x7ffce2f78cd0, 0) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=5, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=62, events=POLLIN}], 4, 0) = 0 (Timeout)
...

Not sure what my option is at the moment. Should I interrupt the upgrade, or wait another few hours?