If your separation has not been made legal a challenge is possible. If it has been made legal clauses may have been written into the Separation Agreement excluding the other party from their entitlement to inherit.
Check your Separation Agreement to make sure that a legal clause has been included. Divorce is the only legal proceeding that removes the legal right share of a spouse, although even then limited exceptions may apply if challenged.
A former spouse can claim that proper provision has not been made for him/her and may apply to court for a share of the deceased's estate within 6 months from the date of Grant of Representation. Reasonable attempts should be made to notify the former spouse of the death. A share will not be given to a former spouse who has remarried.