Hi Alistair,
I am Dave, I will help you with this.
Restart your PC, if you cannot access the Start menu to shut down, on your Desktop, press Alt + F4 to open the Shut Down dialog.
Start your PC, just as Windows attempts to load (spinning dots), press and hold Power Button for 5 - 10 seconds to perform a Hard Shut Down
Do this twice
On the third start Windows will boot into the Recovery Environment and from there you can access System Repair, Safe Mode, Command Prompt... etc.
Go to Troubleshoot - Advanced Options - Reset this PC
Choose to keep your files and apps, just your files or to keep nothing
Then start the Reset