https://www.mig-welding.co.uk/forum/threads/m-w-automatic-centre-punch-repair.83003/
this is a post by Seadog that has the exact same punch as mine 280, the part missing from mine is the half moon spring plate seen in the top photo, not a bloody ball bearing that I thought back in 2016, will see how I go from now, he has a good description further down the page of how it works.