What would be reasonable for me to do to make sure the client is aware of their duties under the CDM Regulations?

This depends on the knowledge and experience of the client, and also the complexities of the construction project. As a designer, you need to have a knowledge of client duties as they affect the project, so you can give the client proper advice. You need to let the client know you cannot begin work until you have made them aware of their duties. You also need to be reasonably satisfied that the client has understood the advice you have given.