I made the mistake of copying in a low-level character, but Ive levelled up a lot since then. I would like to delete my character out of the tribble, then re-register as my fully levelled character. How can I do this, please?

just like you said - on tribble delete the character (think there is an option on the character screen)

and then go through the character copy page again. remember to log out after you delete the original character.

If you copy the same characteragain it simply renames the new one to "Name 1".

Are you sure you deleted the previous character as suggested up above? Sound slike you may have.