Help with inheriting from MembershipUser

Hi all
I'm working on inheriting from MembershipUser in order to implement a custom user store.
At the moment, most methods remain un-implemented, but those I'm writing I'm testing as I go along.
I have implemented ValidateUser() and as a result a call to Membership.ValidateUser() is correctly returning true or false according to whether the supplied username / password combination exists in the data store.
I have also implemented both the GetUser() methods which load a user from the database. However, when I check User.Identity.Name on a page, I get nothing.
Can anyone tell me what I have to do in my MembershipUser-deriving class to get User.Identity.Name to return a username?
Thanks
David

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