I was reading one of those old antiquarian books on holiday in Anglesey last year where according to the one of the chapters, there were 2 carnedd (heaps of stones) near to Brn Celli Ddu up until the mid 18th century or so (when they were apparently broken up to make, of all things, a field wall and bridge...see pages 17-18 of E Baynes below....unless I'm reading it wrong that is, though the guardian article seems to agree). Along with the recently discovered rock art to the ENE, the site could have be a more important monument than it is currently considered. According to the book Bryn Kelli (old spelling) was once called Llwyn Llwyd.
E N Baynes The megalithic remains of Anglesey can be downloaded free here...
https://archive.org/details/cu31924029933011
The book lists loads of sites/tombs lost in the last coupla hundred years.
If you look at the Bryn Celli from Google maps satellite image, there's an interesting almost circular field enclosure very nearby that is unlike other field eboundaries in the area. Probably nothing, but it's curious at almost 550 meters across.
I suppose the pinnacle of Gladman's idea would be Maiden Castle. A site of 'importance' since Neolithic times, ulta complex hornworks at the entrance, and a multitude of banks around it's circumference.
