There will be some heat damage immediately beneath the bonfire. The soil structure will be baked and the humus content destroyed. If the bonfire is only going to affect a very small percentage of the plot I would go ahead with it and mix the 'damaged' soil in with the rest of the plot, once it has cooled down

The ash will be great to dig around your fruit trees!!!

Or simply into your garden

I was going to say that the benefits of the wood ash would out way the damage done by the heat and it would be, overall, a good thing to do.

this time of year, if you're growing nothing there, go for it!

Dig it over well afterwards, I can't see there being any harm done. Don't forget to toast marshmallows on sticks with some friends or neighbours over the bonfire! : )

sounds just the type of place you could find a hedgehog, please check