The laws relating to Wills treat same sex couples the same as unmarried heterosexual couples, who are called ‘domestic partners’ in the legislation.
In working out a partner’s entitlement, there is no difference to a partner’s entitlement under a Will, or under intestacy (where there is no Will). If a person dies without a will, leaving a spouse or domestic partner, but no children, the whole estates goes to your partner. If there are children, the children receive a share. Similarly, a same-sex partner has the same rights to challenge a Will as a heterosexual partner or spouse.
The difference for same sex couples being unable to marry is relevant to revocation (cancelling) of a Will. Generally, marriage revokes a Will. Divorce revokes an ex-partner’s entitlements. If a couple doesn’t marry, any Will they may each have already made, before commencing their relationship, remains valid. Similarly, if their relationship breaks down, the Will is not revoked.