def find_named_image(client, repository, tag):
    """
    Find the most recent Docker image with the given repository and tag.

    :param repository: The name of the image's repository.
    :param tag: The name of the image's tag.
    :returns: The unique id of the most recent image available, or ``None`` if
              no images were matched.
    """
    matches = []
    for image in client.images():
        if image.get('Repository') == repository and image.get('Tag') == tag:
            matches.append(image)
    if matches:
        matches.sort(key=lambda i: i['Created'])
        return matches[-1]['Id']