Yosemite upgrade - stuck on continue screen

[cuneus]

Yosemite upgrade installed without errors - logged in as normal and then presented with Monochrome incompatible app screen (Kindle) with options of Back and Continue

I cannot get past this screen having tried to click on both options (have also tried arrow keys, tab, enter)

The mouse pointer can be seen

If I press power and then again I can login in again but same screen is still present

I have tried plugging in an external mouse

Any suggestions appreciated