Apologies to the previous posters, but this isn't a "technical" matter.
Building Control . They have no actual "control" over something like this.
And, even if the manhole came under the remit of the Water Company, they would simply take down that part of the fence if they needed access. (It would be a different matter if the manhole was built-over and allowing no access at all.)
I guess rebuilt knows this. That's why it's in the Law section.
You can write to them to show you're aware of the situation, and that you object, but finally, legal advice is essential.
It's not what people want to hear, but it's the way to go.