Peter and all,
the above photo and description has solved the riddle how your pivot pin got dislodged and the gelcoat chipped:

The centreboard has lateral play, the pivoting pin has none. Through wrongly fastening the pivoting pin to the centreboard, the centreboard took the pin laterally out with it, until one side of the pin left its seat from under the keelband, and jammed there, while the other side of the pin cocked up and chipped the plastic.

So this was caused by that assembly error at the yard to let the pin rotate in the keel seat instead of in its centreboard seat.
Your proposal to secure the pin to the centreboard with a sunken hex would have done the exact same BTW.
C.

P.S.
And there is one other not-so-nice detail to be seen on your photo: They still first carve out a recessed seat area for an outside-hull flange selfbailer, and then mount an inside-hull flange bailer there instead. Silly.

Through the knowledge of members and discussion on this very usefull forum as  Claus has stated the riddle has now unfolded. When i originally discussed this matter with Matt he asked if i could see the black Silkaflex under the band and i replied yes. My assumption was the pin was fixed to the centreboard and the centreboard strip and it was supposed to swivel under the keelband strips. I have checked this morning and the pins will quite freely move in and out of the keelband strips within the constraints of the slot width of the centreboard. When i knocked the pin back into position i did need to tap quite hard so the centreboard friction on the pin must be considerable as no friction is being applied from the keelband strips so is unlikely to swivel there if i fix the pins under the keelband strips without investigation. So if the centreboard pin is not Silkafexed to the centreboard strip and centreboard the friction must be coming from band against pin against centreboard.

Matt has written to me and has taken full ownership of the problem and between us we will i\\\'m sure correct this one way or another. The good news is this is unlikely to be a wider issue for other boats so no immediate concerns and that a small engineering change by way of the bolt fixing against screws on the 4 keelband positions will futher guard against pin lateral movement.

To summarise, yes i had a problem, Swallow Boats listened and promised to help, due to practical distance problems the boat was not taken to the yard for inspection so relied on discussion and pictures to identify the issue, my assumption on the mechanics was incorrect and the forum discussion brought this to the surface. Additional thought to the pin fixing has resulted in an improved arrangement to be incorporated in new boats.

Thanks to Swallow Boats and forum members for all your help.