Stag beetle maybe?

"Sawdust" on fungi on decaying cherry tree trunk

I am finding the piles of "saw dust" down the sides and at the base of my old, dead cherry tree trunk are growing!   Seeing some hornets building a nest in an old tree in woods at the weekend prompted me to mention this to a game keeper.   He said it might be stag beetle larvae eating their way into the trunk to hibernate.   I looked for holes but can't see any....   It really looks like someone has been sawing branches off - but they haven't!