Thanks for your replies. Looks like they are using a generic name "Emperor" for one of the "Imperator" group of carrots. Could actually be any variety in that group.
Seems to be 5 or 6 groups of carrots mainly classified by length - Nantes, Chantenay & Imperator are the main ones we see in the shops. So they could actually be any variety. They should really be more specific as they are now with potatoes.