I am running Windows 10 and yesterday Windows Defender (which used to be disabled by Avast) became turned on and fully functional. Avast is also turn on and fully functional.
Is it advised and safe to run both Avast Free and Windows (10) Defender?
If Defender has turned itself on then I’d suggest turning it off, running a repair of Avast followed by a reboot might cure it or you’ll have to disable it manually.
On Windows 10 it is Windows that turns off Defender if a third party av is installed.
avast turns defender off in Windows versions prior to 10.
That’s why I suggested an Avast repair, it might make Windows re-recognise that Avast is installed 
Many thanks for your replies, you are very kind.
The problem has now been resolved as Windows Defender has now turned itself off.
Great what steps were taken to fix it?
Defender shuts down when you install a third party AV.
It can be set up as an on demand scanner for a second opinion when checking a file:
http://screencast-o-matic.com/screenshots/u/Lh/1487165063330-43010.png