Thanks Sven,
Sorry for not replying back sooner - I had an initial look when you first posted and couldn't work out how to do your suggestion, which is so seemingly something so simple.
Just been able to sit down and work out that in my company's installation of the software on our macs the technicians must have turned the feature off in the IPTC stationary pad.
In case anybody else has the same problem:
Go to PM Preferences - Accessibility - Customize IPTC Stationary - Then in the top box that's called "Left Side" scroll down to the bottom of the "Field" and "Field Label" choices where you will see "Tag"- Make sure the boxes for "Enable" and "Visible" are both checked (mine weren't) and then when you come out of Preferences and open up the IPTC stationary pad again you will see in the bottom left a "Tag" option has appeared where you can choose "Untagged" if when you ingest you want PM to untag all the images you tagged in camera.
This in my case would be every file left on the CF card as I tag every file I want to keep and keep doing erase all to get rid of everything else. It's particularly useful with the new Canon 1DX's if you hammer away at 12FPS and end up with a lot of similar looking pictures.
Thanks to Kirk and the PM staff for integrating all these great new features into PM5