Ultrabook XPS 13 doesnt recognise Dell Ultrasharp 2713HM monitor via displayport cable

Hi

I cannot seem to get my XPS 13 Ultrabook to recognise my Ultrasharp Monitor.  I have them connected via a mini DisplayPort to DIsplayPort cable.  Monitor input set to DisplayPort of course.

HD Intel Graphics panel only shows built in monitor as as option.  No external monitor pickead up.  I also tried Fn F1 to force switch but my Laptop refuses to recognise.

Am I missing some setup stage?  Any additional driver I need to install?

My laptop is running Windows 10 (upgraded from original installed Win8).   Intel Graphics 4000.  

Thanks