ISABEL LETHAM   History can be a slippery beast, to a large degree we only know what we know, in time historical focus can become quite tight ignoring a broader, more detailed past. As time inserts distance between events and recollections, things can get lost, other elements may be added, and stories can change.   For a long time Isabel Letham was celebrated as Australia’s first surfer. The story of a 14 year old Isabel being plucked from the crowd and invited out to surf tandem with Duke Kahanamoku has been romanticized and repeated for a very long time. This particular tale has become rusted on as an Australian surfing foundation myth. And to a degree it has become a tale which has grown in the telling.   The thing is...