The tombstones date from the 17th century and were once housed in a nearby chapel that pre-dated the construction of the present day building.

translation - (isn't the internet wonderful?)
Should be "Nunes" and "Obiit die 21 Iulii anno 1673 / Requiescat in pace": "Here lies Dr Blasius Nunes Manhans (= Bras Nunes Mañas), Portuguese Physician to the Queen of Great Britain (= Catherine of Braganza, Charles II's wife), died on 21 Jul 1673, Rest in Peace."