Using imac display when venue pro docked

Hello,

I have the Venue Pro 11 7130 and the dock. I am trying, and failing to show the display on my 2014 iMac. I have tried mini DP to DP, HDMI to DP convertor. Mini HDMI (side port of the tablet) to HDMI via an adaptor into the Thunderbolt port on the iMac.

Curiously I can show the display on a PC monitor via the HDMI port on the dock.

Anyone have any ideas as I am all used up?

Thanks

Alan