The Gwynedd Archaeological Trust record has more information. The site appears to be in someone’s garden:
The small megalithic burial chamber (SH 51908275) at Benllech was excavated on behalf of the MOW in March 1965. It was previously unrecorded and was revealed when new sewers were dug nearby and the site cleared for the erection of a bungalow.
No prehistoric finds were made and the only reason for regarding the main chamber as a prehistoric burial chamber is its structure, which is comparable with the Lligwy and Glyn burial chambers. The annexe and field wall are relatively modern.
Not entirely convincing small low ‘burial’ chamber. Capstone perforated (natural limestone pavement). Now in garden of Drws y Nant and in built up area. Capstone supported by uprights and blocks at west side and by recent concreted pillars at east. Stands on gently sloping hillside above the coast. 2 large orthostats – one 2.2 x 1.2m stand just to south. Cadw recently reclassified this as a ‘goose house’ after F Lynch – this doesn’t seem to be a prehistoric monument as it stands – the chamber is very low and could not have been deeper because it stands on bedrock. This could have been disturbed at the time of the sewer construction – a considerable depth of bedrock would have been removed.
