I believe they have to have been unaccounted for for seven years. I'm pretty sure the person seeking the declaration would have to produce a considerable amount of documentary evidence as to what searches had been undertaken to try and find them, including whether bank accounts had been accessed, police search records, Salvation Army and Missing Persons bureau, etc. I believe the government waived some of the more onerous requirements for those who were missing, presumed lost in the Tsunami a year ago, but if somebody goes missing in normal circumstances I think it would involve a long wait.
Normally you have to wait seven years but they have tried to waive this in some cases of people missing after the tsunami. But you will have to go through a legal process because insurers etc won't pay out and if property is jointly held you will be hamstrung. Solicitor is the only way I'm afraid.