With a heading on one checkbox
One checkbox does not require a fieldset and therefore does not require a legend. However, if a heading is supplied, a fieldset will be included in the component and the heading used as the legend, as shown.
How it looks (preview)
How to call this example
<%= render "govuk_publishing_components/components/checkboxes", {
name: "agree",
heading: "Please tick the box to agree",
items: [
{
label: "I agree",
value: "agree"
}
]
} %>