Sam ..
You mentioned clean install but not which OS X is installed.
Just to make sure, the Numbers app minimum requirements are OS X 10.10.4 or later. The v10.10.5 update is available.
Make sure your OS X software is up to date.
Click your Apple top left in your screen.
From the drop down menu click About this Mac
Make sure the Overview tab is selected then click Software Update.
Your Mac may restart several times during the update. That's ok.
Then try downloading Numbers.
If you still have problems ...
Apps such as Little Snitch, Growl, and MacKeeper, can prevent access to the App Store
Disable anti virus software if installed
Check the firewall in System Preferences > Security & Privacy > Firewall > Firewall Options
Make sure Gatekeeper is enabled
Open System Preferences > Security & Privacy then select the General tab.
Make sure either Mac App Store or Mac App Store and identified developers is selected.