CDD must be undertaken because it is a legal requirement.
The Anti-Money Laundering Guidance for the Accountancy Sector includes further details on this area in Chapter 5. The purpose of client due diligence is to know and understand a client's identity, the way in which their business operates and the sources of their funds so that money laundering or terrorist financing risks can be identified and managed. CDD is not only a legal requirement, it will help you to make informed decisions aboiut the client's professional standing and acceptability.
Clients are generally used to providing CDD documents as they are also required by other regulated businesses such as banks and solicitors. Of course if you know a client well it may mean you already know a lot of background information about them which you can note down as part of your client due diligence. You should, however, also be careful not to turn a blind eye to money laundering or terrorist risks because you know or are related to someone.