Worth checking if your anodised aluminium mast step (the metal plate on the fore deck under the foot of the mast) has any holes elongated by the D shackles (used to secure the main and jib halyard blocks). The owner kindly let me photograph the assembly and it appears the D shackle was pulled through the aluminium in the course of normal racing. I don’t believe the boat is more than a few years old, so would not have expected the metal to give so easily. I believe it has raced in two Nationals.

 

While in this area, you should check the jib and main blocks are not twisted or bent. The D shackles do not allow the Classic Harken block to line up with the Spinlock. On my boat I’ve replaced these two blocks with the Harken Carbo Lites which you secure with several loops of Dyneema. This allows the block to twist around more and align with the jib halyard going through the Spinlock. This prevents a lateral load on the block which means the cheeks of the block take the load evenly and stay together.

 

Here are a couple of pics of the failure you are trying to prevent.  Apparently it goes off with a dull twang, the boat stops pointing, and the halyard load tries to pull the Spinlock off the deck (which is only secured by penny washers under the decking).