This is a very rare species which is not only endemic to Wales but also endemic to Gwent. It was discovered many years ago but only finally got its present name in 1978. It is restricted to only a handfull of sites in Monmouthshire. One site where it was recorded in a recent survey (1998) is Ninewells wood.
The scientific paper which tells you all about this species of Blackberry, was written in 2000 by Randall and Rich, if you would like to read it in its entirety then click http://archive.bsbi.org.uk/Wats23p317.pdf If not then here is the abstract to this paper which summarises what it is all about.
Randall, R. D.; Rich, T. C. G., 2000: Conservation of Britains biodiversity Rubus trelleckensis , Trelleck Bramble. Watsonia. 23(2): 317-322, Ust
Rubus trelleckensis is a rare endemic bramble found only in one 10-km square near Trelleck, Gwent (v.c. 35), Wales. It is a perennial which flowers from mid-July to mid-August, and sets abundant fruit but probably does not spread vegetatively. A field survey was carried out in 1998. Five small populations were found, all in locations which were probably at one time either open heath or open Birch-Oak woodland but are now either conifer plantation, or conifers mixed with broad-leaved trees. Plants were most frequent on acidic podzols in sunny but sheltered spots on level ground. The main threats to its survival are change of land-use from forestry or changes in forestry operations. Seeds have been deposited in the Millennium Seed Bank at the Royal Botanic Gardens, Wakehurst Place.
In the paper it says that this bramble does not seem to grow by producing runners which then root at their tip and thus spread quite rapidly, instead it seems to grow as discreet individual bushes which are presumably the result of one seed germinating in that spot. There are other quite complicated characteristics which can be used to identify it such as having no glandular bristles on the flower stalk. Not the sort of thing you would immediately notice, perhaps a little more easy to spot is that the flowers are white, whereas many brambles have a slight pink or purple colouration.
Anyway I am now on a mission to find this rarity and hopefully I might find it growing at Catbrook wood. What may well happen is that after the clearance then it could be dispersed to the area by blackbirds or even a human collecting some seeds from a nearby bush and planting them?? When I find one I will photograph it and post some photos of it on this post. For now you will have to be content with the photo of a bramble which I took today in Ninewells wood so it could be the Trellech bramble but it is probably not.
Finally many thanks to Mack who alerted me to the existence of this plant..