I now faced the tricky problem of removing the front of the mast. I could see no option other than cutting it out.
So I set the circular saw very carefully to cut the exact depth of the rebate in the side staves and ran it repeatedly along the length of the mast while adjusting the fence to effectively excavate the front stave.
This technique worked well. I didn't take any pics - I imagine I was too preoccupied with not wrecking the mast to bother with the camera!
This was the outcome. In this pic I am cleaning up the rebates and hacking out the excessive squeezed out resin.
This is the interior of the mast, close up.
Next is making the new front piece!