get Jane's will as 1st thing - that might name cousins etc - will be on Mormon microfilm - likewise look at witnesses to marriage (again on mormon microfilm)
deeds might give additional details re John - eg there is a sale in 1808 by a John Clark (SSS Onc Oct 1808 #15) - as he was a trustee of the Congregational chapel the trust deed might well give some detail - however these require searching at Manx Museum