Thanks again for everyones help. Just couldn’t get it to work so re-scanned the internet with different terminology, found this, works great!
myParent = thisComp.layer("master");
myProp = myParent.transform.position;
value + (myProp.value - myProp.valueAtTime(0))
You have to add keyframes in order to see this work. If you move you “master” null around on frame 0 nothing will happen – it’s the keyframes further down the timeline where the magic happens!