Doc­u­men­tary about how Tris­tan da Cun­ha, the most re­mote in­hab­it­ed is­land in the world and a sev­en-​day boat trip from Africa, could hold the key to un­lock­ing one of the great mys­ter­ies of mod­ern medicine -​ the ge­net­ic ba­sis for asth­ma.