The name Casson is not Manx - I suspect a misreading for Callow - there is a will of Jane Scarffe in Lonan 1836 (on Brian's site) - summary d Oct 1834; petn by dau + exec Jane;Thomas Scarff + John Scarf next of Kin did not appear;
will - son Thomas.John; dau Anne w/o John [?], sister Ann Hogg; dau Margt Scarfe;husb Philip; I have 'pencilled in' jane Callow as wife of Philip with children :
WILLIAM Lon 17870517
ANN Lon 17891018
JOHN Lon 17940501
JANE Lon 17920122
THOMAS Lon 17990609
MARGT Lon 17961030

I'm afraid I have no record of marriage but there are still some years missing - any likely marriage would be around 1786 - I'm missing 1781,2,3 & 1786 amongst others