Got it working.
I wish I could succinctly explain how, in the event that someone else has the problem. But it was ONE of these steps that did it, I just don't know which one.
I DID have previous version of the BB DM on here from a previous 7250. I also took the advice of jeremyckitching (above) and it didn't work.
However - in the end I:
1 - Removed the Blackberry software.
2 - I then rebooted.
3 - I then spent a good 30-40 minutes going through the registry looking for references to "RIM" and "Blackberry" and clearing out the relevant ones. I went and ensured that I physically deleted all the files from Program File that were related.
4 - Then I went and repeated the advice from jeremyckitching (again) through device manager.
5 - I then reinstalled the latest Blackberry software (4.2.2SP2).
6 - Rebooted.
7 - Reset the device again (removed the battery).
8 - Started the DM software and connected.
It's been an INCREDIBLY frustrating experience, countless removals and reinstallations and playing with different software versions. Hopefully this will save someone else some of that frustration and now I can put the bad memories of the 700wx and the initial 8830 issues behind me.