Hi Ed,
Looking at FamilySearch website, it seems that you are not the first to search for this elusive marriage. The IGI is pretty well covered for Kendal, so either it could be omitted, mistranscribed, or may they were married elsewhere or maybe not at all.
If Sarah was on the 1851 census, where was she born? Could they have married in a neighbouring parish? or even Gretna Green?
Kendal Record Office will do a free 10-minute search (if you ask them nicely) in the records of the parish church Holy Trinity, so this could eliminate omission & mistranscription.
Hope this helps.
Chris