Where my parents live in Hertfordshire, it is the case that you are allowed bonfires after 6pm. I'm not sure whether this is just a council thing or nationwide, but we had leafets about it a long, long time ago and our gardeners who come and mow my parents lawn were aware of it.
Trouble is still, that on a nice summer evening, you probably want to be outside way after 6pm. We used to have a neighbor who always started a bonfire the second the sun came out. My parents had a word with him about and he did them a little less frequently than he used to.
I'm pretty sure that if it's past 11pm it's certainly unreasonable. You could also claim it's a danger because it's actually late, dark and there is a chance that it could be a fire burning without anyone to control it, and certainly less people noticing it if it was out of control.