I often advise clients on matters of this nature. Lawtel or the Withers precedents can assist on the drafting front. The main point to consider is the definition of separation of the cohabitation. An alternative option could be to create a lifetime or Will trust, but this will depend upon the value of the client's estate and the client's view on personally paying, or their estate paying tax.
In terms of the issues to consider, you should look at (i) potential claims on separation; (ii) the assets that would be divided on separation and how this would affect the provisions of the Will; and (iii) how you can protect from those claims within the Will and through other lifetime planning.
Aside from making provision through the Will, consider asset protection through lifetime planning as an alternative option that would leave the client and those the client leaves in control of their estate to decide who receives entitlement on the client's death.
I hope that this helps.