Thanks - I appreciate the warning. I understand it's also flammable, so I'll toss a match in first just to clear any that's in there.
I'm joking :) This is fortunately quite a shallow chamber - I can easily reach the crack whilst keeping my head well outside. There are much deeper manholes...
Thanks for the replies.
It looks like Milliput is a sort of epoxy resin and I'm certain I have some of that kicking around. That certainly seems like an easy fix and the joint is so small I think it's going to be a lot easier to work with than cement so I'll give it a go.
I'll be sure to...
The photo below shows the chamber under a manhole at my house. There's a horizontal channel formed in concrete (perhaps there's a clay half pipe under it?) that is connected to a clay pipe leading to a toilet.
That connection appears to have been sealed with something like mortar or cement...