Redirect problem on custom domains

I've got WPMU going with a root site id=1, buddypress on id=2 and some subdomains running on one of three subdomains:
1 =
2 =
3 =
4 =
And these are working fine. But when I try and add custom domains using the domain mapper plugin, e.g. id 5 =, when I browse to I'm instantly redirected to site 2,
Any ideas? Not sure if it's related but I'm using nginx instead of Apache (in case it's a rewrite rule problem?)