Managed to over write my legal version of Photoshop.

The message doesn't mean that you've downloaded anything wrong. What it means is that it can't check your subscription. If it can't check your subscription, it goes back to being a demo until it can check again. So the first thing is to check that your subscription is still running.  

Which version of Photoshop do you use?

Which operating system do you use?

If you have a subscription I would try an clean reinstallation done by the steps below: 

• uninstall everything cc including preferences, https://helpx.adobe.com/creative-cloud/help/uninstall-remove-app.html  
• then uninstall the cc desktop app, https://helpx.adobe.com/creative-cloud/help/uninstall-creative-cloud-desktop-app.html  
• clean your computer of cc files per http://www.adobe.com/support/contact/cscleanertool.html  
• restart your computer (don't skip this)  
• reinstall the cc desktop app, https://creative.adobe.com/products/creative-cloud.  

if you're unable to install the cc desktop app at this stage, use an administrator account (solution 4 here, https://helpx.adobe.com/creative-cloud/kb/creative-cloud-missing-damaged.html)  

use the desktop app to install your cc programs/trials