Google didn't help (probably too easy a problem for it) so I thought I would right this up in case anyone else was suffering.
I've created a content type, here it is associated with a list in Sharepoint Designer:
But how do I make it default? I finally spotted the option after clicking through endless options in SP designer - the trick is to highlight the content type (and not click on its name to see the content type editor!) and the look at the ribbon:
Click on this and we have a new default:
I hope this helps some other Sharepoint 2010 newbie.