To anyone experiencing this, it is a bug in Win 8.1 which I can replicate at will.
Reinstalling your system as suggested by MS engineers will not fix this.
To force the bug to appear:
1. Grab a desktop Icon (recycle bin, any shortcut) and move this up to the top left hand corner without letting go of the mouse button. Drop shortcut anywhere on desktop. Without an Icon the mouse pointer would initiate the recent apps.
2. To check that the first step has worked, try and invoke charms by moving mouse to top right hand corner. If Charms do not appear then the bug has been triggered.
3. Now move mouse over start button (no click), move away then back again. Now the start button is locked. If you unlock the taskbar you can move it to any other position (top, left or right) and the start button works. Move taskbar back to the bottom and the start button still does not work. Only way to get it working again is to restart window explorer in task manager.
To avoid this happening all you need to do is disable recent apps corner.
1. Right click desktop.
2. Click Taskbar and Navigation
3. Click Navigation tab in the properties window
4. Uncheck "When I click the upper-left corner, switch between my recent tabs."
Now the bug cannot be triggered as above to make the start button stop working. You have to remove this feature which is pretty hopeless on a desktop anyway. Its better than restarting Window explorer every time.
I can replicate this on any desktop or laptop, fresh install or old. Not sure about any touch screen machines. I can also trigger this in VMPlayer\Virtual Box and the Modern IE VM images that MS provide.
Now the MS engineers can replicate this on their machines, and maybe if we are lucky they will FIX it!!!!
MS please acknowledge that this is a bug.
Maybe the issue is due to the windows button not being redrawn on the screen once triggered as it still works when taskbar is moved (graphic card drivers). Could not find anything use full in the system logs.
Just noticed also that the charms also breaks if you drag an Icon up to the right hand corner. Once triggered Charms stops working, start button is not affected with the above work around configured.
MS please read this and reply.