Hi Red,
Red wrote:

It would be a fantastic feature if in the 'Partners' window you could add custom columns as well as the email address and name.

You can add the 'Partner Email' and 'Contact Name' columns to your work area through the 'Edit Workspace' menu (please right-click on the header of any column to activate it). If you click on a partner, you'll be able to edit data in these fields.
There's also the 'Notes' column you can use to set custom tags to partners in your project. In the future versions it'll be made possible to create custom columns based on your preferences.
Red wrote:

It would be a great facility to add a twitter profile ECHO function, and a address / tel other contact details row to make emails appear more professional.

If you would like to show your own contact details and profile links in e-mails you send to partners, you can add this info manually to your mail template through 'Preferences -> Email Management -> Edit Mail -> Email Templates -> Edit -> Template'.
Red wrote:

Is there a way to fill in some custom data and replace it for each email already?

Sure. All e-mail templates are fully customizable. We can send you the list of variables you can use if you create a ticket for it.