Altium: Pouring Polygon attached to a specific net class for direct connections
Found out how to do this. You have to attach the net to to net class in the schematic and then take it though to the PCB. I did find tutorials on how to do this, but they were old and things have chnaged in v19 and above (maybe earlier). 

So:

Place | Directive | Paramater Set 

Attach that to the net. 

Go into the properties of this placed Parameter Set, name it something sensible, then under classes click add and name the class to whatever you want. In my case, Solid Pour. You can copy and paste this to any other nets that you want pouring in a solid plane. For these repeated channels it makes them all part of the same class, which is great. 

Then, update the PCB and add a Rule (Design | Rule) in the Plane Section, under Polygon Connect Style and you can simply say when the first object matches NetClass and the new net class will be in the drop down box. 

It works exactly as I'd expected it to work then. 

Obv, really...