You are certainly in the right area. I would have thought the marriage should be nearby. I believe most marriages took place at the Parish church (Windermere or Kendal), but if the illegitimacy is correct, they could have married at one of the smaller churches. Even though there is no IGI entry for the marriage, it would probably be worth checking Hugil, Staveley, Witherslack, Winster, and Troutbeck as well.