Blocking disused drain branches

Hello

Someone has built an extension on my house over the top of a previous manhole cover

One drain branch hasnt been blocked off , and it is a branch off a 4" pottery type drain.

The branch is about 6 foot from another manhole cover, and about 3 foot below ground level.

How can i block off this branch - is it possible to pump concrete there through a nozzle, or put in a plastic drain liner? (at the moment it is possible a bit of fluid can go up the disused drain and also i sometimes get rodents finding their way under my house - i dont want to dig up the extension and ruin the floor and all associated damp courses, plus im not exactly sure where the branch is, so might not find anything)

Thanks

how do i reach the branch though - it is about 6 - 10 foot away from the manhole

lol thanks! Im looking for a clever way of blocking it either with pumped conrete round a corner or a liner

To be honest i think i am going to insert as thin a pipe as i can get hold of and chamfer it to cause least blockage

Worst comes to worst i can always pull it out again ! (and it will only cost a few dollars , and potentially the rats are blocked