land records provide a good basis - in 1763 his in-laws settle a house + garden in Castletown on him after their deaths in lieu of £40 promised in a marr contract (this contract does not appear to have gone thru a court thus presumably involved no land)
His widow however leaves more land than this and John had sold some to his eldest surviving son Wm just before his death - how did he acquire this land ?

there is a John Cubbon who buys a small house in West St in 1759 - maybe this was him but this house is not what he sold to son nor what his wife left