What Fleecewife says. Talk to the owner of the rest of the flock, find out when he/she's gathering them next, turn up and catch yours and stick them in the back of your car, so there's no panic about timing it with a horse box.
I carry goats and the occasional sheep like this, seat down, dog guard, or in my case, a piece of weldmesh tied in place with baler twine to the front seat headrests, tarpaulin over the folded down seats, and then some blankets to absorb any accidents. Remove tarpaulin, blankets into washing machine when journey over.