Confidence: It's likely to be this, but I can't be certain.
Notes: Large black ant with reddish thorax making a large domed nest of pine needles. I believe that F lugubris and F aquilonia, whose UK range is basically in Scotland, do extend down into Northumberland but this site is south of the Tyne. Those species have a hairy thorax which the ants in the picture appear to lack.
The nests which can be a metre high, were photographed at the same location last year.
The ants are very numerous and widespread in this woodland with lots of nests of various sizes.