I'm not sure whether they make allowances for people who are unable to come back for medical reasons? (ie the NatSci you mentioned) I mean, as far as I understood it the historian had two years out because he got bored with Cambridge; so if they'd let him bugger off and come back again I can't imagine why anyone else would be prevented from doing so, especially for more important reasons? (I've nothing against said historian, but it's basically true!!)
edit: arch&anth? Was that in our year? - Henry?