Accessing custom block fields from a template file
This was the final solution.

{% block content %}
    &lt;a href="{{ content.field_link.0['#url'].uri }}"&gt;
        &lt;img src="{{ file_url(content.field_image.0['#item'].entity.fileuri) }}" alt="{{ content.field_image.0['#item'].alt }}"&gt;
    &lt;/a&gt;
{% endblock %}