the comment in the settlement by John Cubbon's in-laws that there was an earlier marr contract suggests to me that john was somewhat older than you think and was already established in the world - there is a John Cubbon s/o the John Cubbon whose will is in 1723 (malew) tho havn't found a bapt (all the ch were ua and I think the unnamed wife was Ann Keig - they married in 1711 + Ann remarried - see her will

I wonder if this is your John?