rachel2, you are not required to give any costs information to a residuary beneficiary - but the Law Society says it is “good practice” and the Legal Ombudsman says “it would be prudent”.
I quoted the advice of the Law Society and a report from the Legal Ombusdman at greater length in this post in another thread:
The consensus seems to be that you should give costs information at the outset to all residuary beneficiaries - whether or not they want it.