Back in the dry of the cabin and with a cabin light on (The boat has a heavy winter cover over it), I slackened the nut to a position where the nut head was the same level as the top of the bolt. Then with a few sound lump-hammer blows, managed to push the bolt downwards. I removed the nut and another hefty blow put the head of the bolt flush with the Oak apron. Note: it is important to replace each bolt in turn to ensure all the keel and apron keeps in place.