Hi!
 
I have noticed that after installing #1709 update the precious disk space on C: is greatly reduced compared to before the update and very much different to the initial win10 update. 
 
Unfortunately my SSD is only 100 GB, so I was happy to see win10 leave my about 40 GB when initially upgraded from win7. Yesterday after the upgrade to #1709 it was down to 4 GB.
 
I meanwhile disabled hibernation mode using "powercfg.exe /hibernate off", now I'm back at about 19 GB of free disk space.
 
Is there anything else I can do to cut back on the things I do not need and keep more space?

The "old" Windows is probably taking up some of the space.  When it upgraded it created a folder with Windows "old" so you could fall back if needed.  
 
If you run Disc Cleanup you can get rid of it.
Here is a link to the tutorial on the tenforums.com.  I would also suggest doing the "Cleanup System Files" option, too, or even the first step.
 
https://www.tenforums.com...anup-windows-10-a.html

Agree with fireberd - if you are running OK on 1709, ditch the backup files.

Make sure "Turn On Fast Startup" is not enabled.

fireberd
The "old" Windows is probably taking up some of the space.  When it upgraded it created a folder with Windows "old" so you could fall back if needed.  
 
If you run Disc Cleanup you can get rid of it.
Here is a link to the tutorial on the tenforums.com.  I would also suggest doing the "Cleanup System Files" option, too, or even the first step.
 
https://www.tenforums.com...anup-windows-10-a.html

This is it ^^^

Windows.old on my system is less than 100M.

Windows.old was about 20GB on mine after the last update to 1703.
 
Windows is supposed to auto delete it after 30 days, but you can recover space early by following the method above.

Thanks guys. I deleted the old files from before the update as I have a full system backup from just before the update anyway

That and disabling hibernate got it back to "normal"