Thought this may be useful to others. My servo stopped working so I thought as I have M16 calipers I would replicate the TR8 set up with larger servo and standard master cylinder. Firstly the problem was not the servo anyway it was the seal between the cylinder and servo, but I thought I would push on. the sherpa servo bolts straight in but the problem is the rod which pushes into the master cylinder is longer and a bigger diameter on the sherpa servo.
found out the rod is just held in with a simple spring clip which can be hooked out easily enough. hook behind the fingers of the clip and it pops out, the rod then just pulls out, to re-fit the clip a piece of tube is needed to push it back in, as I had already fitted the servo to the car I welded a washer to the end to keep it centralized. fitted with a new seal the cylinder went back in and I now have reliable assistance and a lighter pedal effort. I have some photos but no other site to load them to.