A quick update - I fitted the new rest and pressure button tonight and got a guy down the club to help me tune them. Just basic stuff by watching the arrow flight, no bare shafts or anything fancy
Looks like my arrows are a little stiff but with a soft button they fly much better and faster than before. In fact I had to raise my sight by 1.3cm at 40yds and the sight is much more centered than it was before. Also only very occasional hitting of the shelf which I am happy to put down to a dodgy release as it is no longer obviously tail down in flight.

For the moment I think that is more than good enough so will leave it alone and concentrate on my form