Dell XPS 13 - USB-C to Video Out - HDMI and VGA failing with Dell dongle

I have one of the new Dell XPS13s with the USB-C port rather than any real video connectors, and waited patiently for a genuine dell dongle. (I have the one which has 4 outputs - Ethernet, USB, HDMI and VGA out DA200?)

Giving a presentation the other day, I suddenly found that I couldn't output to a screen. I returned home and have since found I can't output to my home screen either. The dongle appears to be working, at least the USB portion of it works fine, but when I attempt to hook up an external display I get the following message:

Try improving the USB connection
DisplayPort connection might not work. Try using a different cable.

I can now no longer output via VGA or HDMI. I've tried numerous restarts, checked for driver updates, firmware, etc.

Anybody come across similar problems? Any obvious fixes I have missed?

Reliably being able to connect to external monitors is more or less mission critical for me.