In the middle of the circle is a hole which needs to be filled with “a kind of flue” or “chimney” made of straw.
“That was to allow a little bit of ventilation in,” he added.
With the straw in the centre, Alan said to keep layering your sand and carrots until you ended up with a little “chimney of straw” coming out the top.
“As you want them, you can go and get them and you’ll find they haven’t been attacked by slugs but that they stayed cool and they’ve not gone wrinkled or dried out.