For anyone who'd like to keep their Local and 'main board' identities separate.
Using buttons at the top right of your screen (desktop version), go to Mumsnet.com then My Mumsnet / My Account, to see if you already have a Local Nickname. If one is shown, the only way to change it is to email HQ at email@example.com with a request for your chosen new name. (You can check the name you want is available, by trying to change your main username but not pressing ‘save changes’).
If you don’t already have a Local Nickname, the first time you attempt to post a message on a Local thread, a pop-up will appear, at the point you click on ‘post’ - be brave! - asking if you’d like to set a Local username.
Once your Local Nickname has been set, it cannot be changed, without emailing HQ.