Sounds like we're practically neighbours! I live near Ilkeston.I've seen a few Z4s around while I've been going around the area, but not many Z3s.
A red Z4 going through Sawley towards Long Eaton at around 1900, but that would have been far too late for you guys, and a black Z4 with no front plate followed me up the A610 from M1 Jct 26 at around 1300 near Eastwood. It may have been the one in post#14, but I don't think so.

