Thanks to everyone for their efforts to sort this out. I have now found the problem ..... viewing the emails on my Samsung Note 4 in portrait format it doesn't display my username, but I have discovered that if I turn the phone around and read the emails in landscape mode ... hey presto it does magically appear in the top black bar after all. Mystery solved ... thank you guys and gals
You might find it easier if you change the First Name on each of your accounts, so the are all different. The first name is displayed when sims are posted out, and on emails, but it isn't used for anything important so it can be anything you like.