Window cill replacement

This started as a small area of rot in one of the corners. But once I started It became apparent that the whole cill had perished.
The exterior walls had been rendered and the interior walls plastered after the window had been fitted. It would have meant either a re- rendered exterior wall or a re-plastered interior wall if any attempt was made to remove the window frame, so replacement was not an option.
The property which is a grade 2 is situated 100meters from the beach. All new work was finished in a marine primer and marine grade paint will be used.