I can't pinpoint Somerset House for you hencarrai but judging by the description of the enumeration district in which it was located on the 1861 I'd say it was between the village of Six Bells, a small village adjoining Abertillery to the south and the southern end of Abertillery itself.
The enumeration district mentions: Pantyarrail, Cwmnanty, Rhiw Park, the Coach and Horses pub (still a Coach and Horses in the area though don't know if it's the original, or still open), and there is still a Pantypudding Road. Most of the names described are still there in some form. The main road from Six Bells to Abertillery is Alexander Road which then becomes Somerset Street in Abertillery. Also too, there was a Somerset Hotel further along Somerset Street. Can find no mention of Somerset House in a street atlas of the area but I think this is the right area.
If you search for Abertillery on Google maps, zoom in and Somerset Street soon appears. Street View is available.
