@mrjj No I don't have a .pro file so I am opening using a .qbs file
@Bagavathi I have done as you said, and that has worked. (i.e. Upgrading gcc/g++ )

In the meantime I also discovered ( I am on Ubuntu ), that using command-line and 'make' works fine for all projects. So I am sorted.
Thanks very much for your help.
:-)
John Montgomery, Glassel, Scotland.

@AlaaM said:

Are you talking about this:
run sudo apt-get install libqt4-dev
?

No, that's Qt 4. @SGaist suggested that you install Qt 5.

@AlaaM said:

No, there's no warning icon next to the kit:
http://i.stack.imgur.com/1H1TX.png

Hmm, the kit looks ok.

Can you describe exactly what kind of project you're trying to create, when you got the "No valid kits" error message?

@AlaaM said:

Yes, there is a yellow warning.
Please see this image:
http://i.stack.imgur.com/gohot.png

This image tells me that you manually added Qt 5.2.1 to Qt Creator. Do you remember where you got this copy of Qt 5.2.1?

I installed Qt from here:
download.qt.io/official_releases/qtcreator/3.5/3.5.1/qt-creator-opensource-linux-x86_64-3.5.1.run

Like @SGaist said, that's the Qt Creator IDE. I recommend you uninstall that, and install the Qt library instead: http://download.qt.io/official_releases/qt/5.5/5.5.1/ (this installer also includes Qt Creator)

Hi and welcome to devnet,

You also need to make it known the Kit.

By the way, why not use the MinGW compiler provided by Qt directly ? Doing so you would already have everything ready to use.