ArchSearch lists 3 relevant items at the grid reference: a robbed Late Neolithic to Late Bronze Age cairn NTSMR-NA6900, a prehistoric cairn NTSMR-NA2382, and a possible prehistoric cairn or extraction pit NTSMR-NA20875. All do come under the SSSI and scheduled monument.
Interestingly this is not mentioned on the MAGIC site and not given a schedule by English Heritage, but it is mentioned in the handy National Trust leaflet 'The Archaeology of Lundy' which writes "John O'Groats is a ruined post-medieval building on top of a prehistoric burial mound or cairn. The building may have been a link to the lookout posts further down the steep cliffs. The cairn is one of a number situated on high points across the island."