Thanks for getting back to me Chris - I can indeed create a new user using the user management page, the new accounts seem fine, I can log in with them, change passwiord etc and they show up on the user management screen with the correct user name and also
as Active and IsInSharePoint.
However, the accounts which I created through the membership request web part have the " i:0#.f:|fbamembershipprovider| " prefix on the user name and are not marked as active or IsInsharePoint.
Looking around on the web it seems that some people are getting the i:0#.f:|etc issue when they upgrade from SP 2007 to 2010 and try to take existing FBA users with them, something to do with the way the Security Token Service hands over to SharePoint.
This makes me wonder if there's some setting in IIS or Central admin which is wrong - I set up FBA by following Donal Conlon's guide (http://donalconlon.wordpress.com/2010/02/23/configuring-forms-base-authentication-for-sharepoint-2010-using-iis7/)
so I'm going to go through that again and check whether I've missed something.