Your post is 19 years old but hopefully you are still on here! I've moved to Lydart recently and came across a reference to the Pecket Stone and this post was one of the only references online I can find about it.

Reading about its location at the boundary of Trellech and the other 4 parishes, I think that places its location at a small parcel of land next to the Trellech-Monmouth road where there are a group of trees and a large stone marker with a plaque marking where the boundaries meet which you can see in this Geograph photo:

https://www.geograph.org.uk/photo/203161

The stone that the council used in the 1980s to fix the plaque to is a substantial size and interesting shape which I'd be surprised if they deliberately moved there just for the purpose of an information plaque. I'd say it was likely a large stone already in the vicinity which the Community Councils made use of. I'm wondering if this could be the Pecket Stone accidentally reused and hiding in plain site??

Nothing solid to back this theory up of course!