The MY07 Z is 1,603Kg for the basic version and 1,623Kg for the GT version according to european specs! Add a couple of kilos for the UK version (down to the folding mirrors and alarm system). That's one heavy mother!
BTW, these weights are according to EU norms, which include a 90% fuel tank, and 75Kg for a passenger and luggage. BMW always quote weights according to this formula. A MY05 Z is 1,525Kg for the basic version and 1,545Kg for the GT version, though I'm not sure if this is according to EU norms or not.
An M5 or M6 is extremely light compared with the Z in terms of what's packed inside (equipment, size, not to mention a 5.0 V10 )