I don't think they're cobnuts, which are in fact hazelnuts. We have hazel trees in the garden and the squirrel deposits the leftovers all around and buried his winter supply; they have a smooth husk.
They could be Cob Nuts, very popular here, and very tasty. There are lots of trees in the road my doctors surgery is in and you see people collecting them when they fall.
Yes, do what EllanVannin suggests. If it’s a walnut, it will be immediately obvious. It doesn’t look like the sweet chestnuts I used to have in my garden - they were spiky enough to need gardening gloves to handle, but they may be a different variety.
I really enjoy making preserves from foraged fruit. Can anybody, please, tell me the name of this fruit which is growing wild nearby, and is it edible? It has a rough surface and the leaves look a bit like a chestnut.