Photos show 8 different individuals. The average nPN of this sample is approx. 2.94 (not corrected for CS). One individual had a mild Aegeritella infection: https://www.inaturalist.org/observations/228126835
Compared to the typical F. rufa of Belgium, the trails of this colony were clearly more populous, with slightly smaller workers. The relatively small mound was largely hidden under vegetation and a (presumably man-made) pile of branches.
Fungal infection of Aegeritella sp. on worker ants of Formica rufa.
See the worker specimens with a hairy thorax dorsum, setae count greater than 12. These workers happen to be infected with Aegeritella sp. fungus:
I found a dead gyne just two meters away from this nest. Based on Seifert's key, the ventral surface of first gaster sternite should have more than 24 setae in lateral view, which this queen specimen definitely has. With no sight of F. polyctena colonies nearby, I am certain that this gyne belongs to F. rufa and is probably from the same colony.