There's no undo. Since you don't describe 'a right bloody mess', you could try:
A) Use the backup that you always create before trying a mass import.
B) If you have a record creation timestamp field, you can do a find on that field to get the most recently added records, check them, and delete them.
C) if your import performed some auto enter steps, and you have a primary key, sort by that key-the records will be in creation order.