I regret I cant be of much help, for although Wiston records are on the Internet and both Careys that interest you were married there, no Careys seem to appear amongst their baptisms. The Careys themselves seem to be predominently located in East Sussex, although there were a few in Storrington about the time that interests you. Hannah bb to Martha 1789, Richard 1789 & Charlotte 1790 to James & Ann, or an outsider, Richard born to John & Elizabeth 1781.
If you have no other option, see what you can find out about James & Ann, who seem by location and year to stand the greatest chance of being the parents of your Stephen.
It may also help if you also disclosed a little more on Lucy, for if she was up to 10 years older than Stephen, there is a Lucy Carey at Funtington on the 1841 census that might be her. Also on that census is a James Carey born c1767, living in the Worthing-Littlehampton area, that could be the James above. Roy G