the PMV needs to go AFTER the last stack - you need to keep both stacks at pressure for the media to work. That might make it easier to do - all you have to do is connect the stacks and you probably already have the outlet to the original coltri stack ?
6-1/8 might be = 1/8 BSP thread to a 6mm tube fitting (its not so bizzare as it sounds having metric and imperial mixed.)
So, the way we have ours is hard 6mm tube between the output of the coltri and the extra stack then the PMV into the 1/4 BSP outlet of the second stack then on to the whip. So you'd probably need some 6mm tube (cheap) and a 1/4 BSP to 6mm fitting, plus the one to go into the coltri which may be 6-1/8 as per the above.
Local hydraulics shop will have all the bits....
Steve