It seems the key bit of info for resolving this is location. Why can you not determine this? What is the exact location given as printed on the NIP? The A10 is a long road.
Presumably as two fully functioning 30+ year olds, with no medical issues, remembering where they changed drivers a week ago will not be a problem.
As they have been down this avenue before, they will know that shrugging their shoulders and saying dunno guv will see someone charged with a s.172 offence. I expect they know who was driving, or will as soon as they can determine the location.