A well-drafted cohabitation agreement should complement and not compete with other legal documents. For instance the provisions of a Will should mirror the terms of the cohabitation agreement so that there is consistency of the common intention of the agreement of your financial affairs.
Another example may be in terms of the division of property upon separation, that due consideration be given to registering a Declaration of Trust, which is then to be lodged with HM Land Registry.
The construction of a Cohabitation Agreement, ultimately carries more weight the more up to date it is, therefore, it would assist to review and amend the terms of the document every 3-5 years to ensure it reflects and addresses your current financial arrangements.