Ok, solved. For future reference, the thread on the original bar end weights for the XT660Z is M16x1.5. I now have the taps for this, if anyone wants to use these, I'm happy to post them to you and you can post them back after you have tapped your bars.

___XT660Z___mods_so_far_____

Akrapovic twin pipes
Renthal RC high Fat Bar with TNT Racing risers
Ohlins rear shock
Ohlins fork springs
Touratech zega pro panniers
Guard-it bash plate
Guard-it rear rack
R&G tail tidy
12v power socket on bars
Touratech satnav mount

To do - hand guards, excel rims (going to 18" on the rear) with stainless spokes