I’d guess at ringless honey fungus. I thought oyster tended to have pretty much non existent stems but having said that I don’t have massive experience of woodland fungi (Lincolnshire isn’t exactly known for it wide swathes of ancient forest )
It's difficult to tell from photos but my final guess is honey fungus. What condition is the tree in?
Take care when felling it, honey fungus will leave the wood really weak. It can be difficult to get a hinge in order to control the direction that you're trying to fell in.on its last legs cutting down in a month or 2 (weather depending)
PS internet really fiiiinf up