Department of the bleeding obvious this one.
You might remember that a few months ago I added a linux partition to my old Thinkpad while it was still on Windows 7.
Now, on a dual boot machine, linux always installs itself as the first option in the boot manager script.
This didn't really matter as windows 7 was no longer supported and was no longer going to receive any updates.
What's more, when ubuntu and its derivatives install updates they always do an os-probe and check what operating systems are on the machine, and rewrite the options in the boot menu appropriately, always putting ubuntu as the first option.
And this all worked well as long as I left the windows partition on Windows 7.
However,the time came when I couldn't delay an upgrade to Windows 10 any longer, so I upgraded the Windows 7 partition to Windows 10.
And not unnaturally, Windows 10 wanted to download and install updates in the background, along with its habit of doing a reboot or two while doing an unattended update.
Obviously, if the first, ie default, option in the boot menu was Ubuntu (or indeed anything other than Windows 10), it would get stuck half way through its update.
Not a good thing to have happen.
The solution is of course simple - always have windows as the default operating system, and remember to change it back after any linux updates ...