U3415W not entering power saver mode (Ubuntu)

Hi - was pointed here from the peripherals forum...

http://en.community.dell.com/support-forums/peripherals/f/3529/p/19659886/20842486#20842486

I have a new U3415W connected to a GTX 980 via the Dell provided displayport cable, using DP 1.2

When the screen is idle it blanks (set via xscreensaver) but the backlight is still on and remains on indefinitely

(I also get no audio, but wouldn't use it anyway)

it's actually entered power saver mode on 3 occasions so I know it does work (it also works on Win 10).. I assume there's some process interfering with this, but I don't know enough about this area of Ubuntu to pin down the culprit despite being fairly ruthless with killing processes. I've played around with some settings on the monitor (DP 1.2, etc.) and also NV-Settings but to no avail

Any ideas? Let me know if there's any useful information I can gather & post

thanks