The borders of copper areas used to be very wide but  recently have been reduced in width so it is harder to find them. In the right hand panel of the PCB Editor, set Copper Area to invisible. Then you can see the thin dotted outlines of the copper areas. Click on one to open its properties in the right hand panel or right click after selecting to open the context menu.

Thanks - that worked.  I thought I was selecting them but obviously not

OK I removed the ground planes but it's left two little white "GND" character symbols similar to the ones inside pins which then create DRC errors.  I've tried selected them to delete them but there's nothing there to select. Any idea ?

Had a thought and selected the whole layout and moved it several inches left and that left these GND symbols behind so now passes DRC.  But they are still floating there.  Weird

@MikeDB, From what you've said, I don't recognise what these symbols are. Could you post a screenshot or add me as a Team member (though I'm guessing that is not an option) to see quite what these are?

Here you go.   Once I moved the design stage left, the red track appeared which wasn't there when the rest of the design was there.  But it won't delete.  There was a second one higher up which also won't go.   If you look close you'll see it says GND in the middle which came from the ground plane I deleted.   

Hmm, What happens if you select one of them and then try to move it? Does it show green start and end vertices with a blue add-vertex dot in the middle? If you select one then press H, do all the other pads and tracks connected to GND light up? Do they appear to be associated with anything in the Design Manager in the left hand panel? What happens if you click on the GND net in the Design manager: does that also high light these stray tracks? If you click on GND in the Design Manager Nets list the grounds should all light up. What happens if you then untick the GND net in the Design Manager? How about making a copy of the whole PCB then in the copy select everything using a left-click-drag-left select box then delete it. Does that leave behind the GND track segments? And does that reveal anything helpful?

It's not selectable - purely a ghost.   The Design Manager didn't recognise it, and once I removed the ground plane there wasn't even a net called GND as I renamed them back to the separate 0v lines I had before. I basically did your last idea only cut and paste it into a new page, leaving the ghosts behind. I do seem to be getting a few oddities, like that IC on the bottom that wouldn't mirror last night but was when I rebooted my PC this morning. Hey but the price is right :-)