The short term fix is to use the offline installer for an older version, install the plugin, then upgrade AD but don't upgrade the plugin.
The serial/key for the new plugin is now supplied by the Solidworks reseller but they didnt know anything about it.
Even though I got it working, I'm back to using import/export step models. I never liked having to use 'managed' projects anyway, there nothing but headaches with that Vault thing.
Once I got the plugin it working, my existing managed projects complain about them not being in the Vault (I guess I should have backed it up before reinstalling windows).
I deleted the SVN sub folders, uninstalled Vault, and recreated the projects as unmanged. Problem fixed. KISS for the win.