You have two options:
1) An exact yellow replica of the tag that's been lost (not necessarily from the same tag company, but with the same flock and animal number), OR
2) Also remove the other tag, and then re-tag with red replacement tags which will have your own flock number on them.
If you go with the red tag, record in your book that the original tag was lost, and that you have re-tagged with your new number UK0[your flock number], [the new animal number from the replacement tags] etc.
HTH!