Hiya Branwen,
The Witches Stone at Spott was added by me a while back;
http://www.themodernantiquarian.com/site/6453/witches_stone.html

Great place to visit.
Re the Slidey Stone at Arthurs Seat- do you know here this was exactly?? I grew up in Abbeyhill just next to Holyrood park and spent many a happy day playing up there. When I saw Slidey Stone at Arthurs Seat, I thought it might be the outcrop of rocks at Hunters Bog- we indeed used these as slides for many years!The RCAHMS have a small publication/booklet on Arthurs Seat, but there was no mention of the Slidey Stone....

Cheers,
Martin