The cache is not located near the church but in that area. Go up the hill in front of the parking lot. You will see the roof of the church and its little tower, when you're near the cache and the view is beautiful. In the cemetery, in front of the church, I noticed the tombstone of Jón Páll Sigmarsson, the world's strongest man in the nineties. I didn't know his tombstone was located there.
Here is info about Lágafellskirkja: The church at Lágafell was consecrated in 1889, although an oratory had previously stood on the site in the late medieval period. Significant renovations were carried out in 1956 and 1979, and today, the church has a seating capacity of 160-180. The area has been inhabited for centuries. During the first half of the 20th century, Thor Jensen, a well known entrepreneur, purchased Lágafell and the surrounding land, from which there is a magnificent view of tge plains below, as well as of the farms Korpúlfsstaðir and Blikastaðir. Mosfellsbær aquired the property in the 1940s and a large part of Lágafell today