While installing the update to Hyper-V RTM the installation would seemingly finish successfully, but then threw an error during the reboot while configuring the updates. To solve this we tried to shut down all the VMs and stop all the Hyper-V related services, but even this did not help. Eventually we completely removed the Hyper-V role, installed the updates, re-added the role and all was fine.
It meant having to reinstall all of the Integration Services components on each of the VMs, but at least their settings were largely still in tact, except for the need to recreate the Virtual Network and set that accordingly on each VM.