Aha, a small problem has been solved today that has been a minor bugbear with Twitter.
When retweeting, it is sometimes necessary to edit or even amend the original tweet to include a comment, an answer, hashtag, or shorten it to fit in the 140 characters.
Discovered today that the protocol of RT is to leave the original fundamentally unchanged, and if you do need to change but still want to ensure that credit goes where it is due you use via@username instead of RT.
Bingo!