Have just seen your query about Nantyderry house.
There is a Nantyderry House At Nantyderry,near Goytre (sometimes spelt Goetre) which is approx. 6miles north of Pontypool South Wales (postcode NP4)on the A4042.
It was owned by Gwent County council and closed by them approx 1993 and ownership transferred to Monmouthshire County Council in 1995 who sold it to private ownership, and is now just a private dwelling.
If you are still looking for information if you can let know what it is I will try to find it or who you need to contact