First real post since being back and it's an absolute cracker! I can't take any credit for it I'm afraid as it was left in the comments of my post about how to get Forms Authentication and SP 2007 working by :
If you want Sharepoint designer and/or infoptah to work... login to your site with the "forms" admin account, and check the "sign me in automatically" checbox...
Sharepoint and Infopath will use this cookie to authenticate.
Thanks Patrick! I know a lot of people were asking this in the newsgroups and it wasn't something I'd got round to figuring out myself yet.