Best practise really does dictate that all stages of the recruitment process be documented, including an analysis of all of the responses to a recruitment campaign, once the position has been filled.
As part of the response handling stage, candidates are emailed a Diversity Monitoring document which they are invited to complete. Whether they do so or not has no impact upon their application, and such responses are completed anonymously.
At the conclusion of the recruitment project, a review is conducted with the results being relayed back to the client if they require them.