1. How do I tell if the walnut tree growing in my proposed foresty bit is a black or ordinary walnut.
2. If it is black, how far away should I plant chestnut, hazels etc, or should I pollard it?

Hi.  Black walnut trees have deeply furrowed, ridged bark; ordinary walnut bark appears smooth. Both have ovate leaves; black walnut leaves are a slim feather shape while ordinary are moe oval.The nuts from a black walnut will drop from the tree in their green husks.  
This is an excellent source for I.D.:https://www.wildmanstevebrill.com/plants, click on black walnut.  The spacing area is prohibitive, it has to be out range of the tree roots, fallen nuts and leaves. https://garden.org/frogs/view/6212/
I didn't know this when starting out and couldn't figure out why nothing would grow in a certain bed.   I have read that squash is not affected by the chemical excreted by the Black Walnut, yet not a seed sprouted when I planted them.  Good luck!

Susan

I'm not familiar with farming where you are, but I imagine that English/Persian (what you called "common") walnuts are more common there, so it's likely one of those. English/Persian walnuts release juglone, but I've read that it's far less than black walnut or butternut.

The effects of juglone seem to vary quite a bit based off of climate and soil type too. Where I live in the Pacific Northwest of the United States, I've never seen or heard of a case where walnuts, even black walnut, stunted the growth of other trees. That doesn't mean it doesn't happen, but I've also heard from people who have worked with food trees for decades that they've never seen it either.

Thanks guys. Looks like one smooth looking trunk to me! Such a relief as I would hate to lose it. It is growing out of our well and is 5 years old now.