What are you trying to achieve in the first place?

I have more than one account I send from and the wrong one is set as default at the moment. Ideally I could do with a dummy account which thus forces me to choose which account - that's how I've got Windows Live mail set up ( and Outlook Express previously!)

Even though you can designate an account as 'default' one in Thunderbird, that doesn't mean it is always used automatically when composing a new message.

You can either select the desired account prior to pressing the 'Write' button, or you can pick the desired 'From' address from the drop-down menu in the 'Write' window.

How to change the default outgoing server?

That's something entirely different. So not sure what you really want.