Yes, of course I already found that panel and the 'sleep' settings was not there - hence my complaint. However I have now found the solution. In the XPS8900 there are two HDMI connectors available, one on the machine case and the other on the video card itself. I've now obtained a suitable cable and voila! Using the direct connection to the video card (a GTX 750 Ti) I now have the sleep option! As well as that the NVidia control panel now opens, it didn't before. Others with this issue should  note that you need a HDMI to mini-HDMI cable, the standard HDMI plug won't fit. Even the mini one is a tight fit, so choose  one with a slim body. It would be helpful if the documentation supplied with the XPS machine made this clear, I only found a pointer in another forum entirely, and that solution dealt with the missing NVidia control panel not the missing sleep option. Dell might also consider supplying a suitable cable with the monitor, HDMI is the emerging standard for this kind of connection.  
This XPS8900 had the usual Win10 power options (sleep, shutdown) until the old monitor was replaced by a new U2518D UltraSharp model. Now the 'sleep' option no longer appears, let alone is selectable. Both the PC and the monitor have been updated to the latest video drivers etc., and the firmware on the monitor likewise. How to get the sleep option back?
XPS8900 + U2518D sleep problem