A confusing genus with four species rather similar looking, not helped by the changes from just appearing to maturity. This species has clamp connections (connections between hyphal cells) which help split it from Armillaria mellea

References: 
Kibby (2013) Field Mycology, Volume 14 (1), page 4 - Key
O'Reilly (2016) Fascinated by Fungi
Overall (2017) Fungi: Mushroom & Toadstools of parks, gardens, heaths and woodlands"