This is a beautiful, though poisonous, fungus - Lepiota cristata. The id was confirmed by the strong chemical smell that gives it its English name, Stinking Dapperling.
I'd visited the plantation, alongside the Steppingley/Woburn Abbey road, to have another go with the Bottolo decoy. I used it from halfway up a tree and soon had a female Muntjac pacing around and barking, followed by a barking buck a little while later. But the most interesting deer sign were these slots which, at about 6cm, probably belong to Fallow or Sika Deer.
These tracks are much easier to identify: Badger!