I forgot my username, what do i do ?
Try going here: http://giffgaff.com/support/lost-password
And seeing if you get an email from them with your member name in the subject feild or message?
Try looking through your emails to see if one of them contains your username. Your registration one certainly will.
If that fails you will need to contact an agent with as much information as possible and they will be able to help you out.
Go to http://bit.ly/giffgafflost