Posted: 27/06/2015 at 12:31
Cauli's are probably the most difficult veg to grow well for home gardeners. The young plants need to be really well firmed-in to produce dense, tight heads which would be difficult in your sandy soil - they are commercially grown in areas with clay soil. I would harvest and use them as they won't tighten up now.
If you grow them again, try planting the young plants as deeply as you can - right up to the bottom leaves and use your heel all around each one to firm the ground around the stalk.