Your elbow should lead
Hi Chas, yes at some point the long awaited video of my revised serve will appear. What the coach I have been working with said, that I found useful, was that in serving when you are ready to hit the ball, your elbow should lead. For me, this makes me focus on the bend of my arm and the use of "hand speed" to go through the ball rather than using the whole arm "arming it through" as I used to. I think this is working.
Additionally, practising Pilates once a week with my PT, I feel that the awareness of the core, and the supporting muscles, secondary to shoulder, enable me to get more power in serving without just using the arm...