Digging to downpipe drain issues (potentially blocked)

[Hobobanobo]

So we had damp issues in the kitchen corner here when moving in. The ground below damp proof was very wet and have had subsidence issues due to clay.

Our main waste drain is to the left of the Image and the toilet pipe drains to it fine. There is another exit pipe in this main drain which I assumed was for the downspout. I had assumed our downspout pipes were either cracked or blocked as I stuck a Hosepipe down along with my arm and found no exit at the bottom, or any water draining to the mystery exit pipe in the sewer.

Assumed great, ill dig it out and fix it. It looked like someone had tried in the past as concrete was replaced here.(I had also assumed a garage downspout drain connects to the one by the house and would head for the main sewer drain.

But I've hit foundations...
Now what? Do they drain the other way? Why when the sewer drain is right here at the back.
I don't know the best plan of action now.

Any help appreciated.

 

[Hobobanobo]

Oh. I managed to clear the crud out the pipes and they do indeed head the other way. Oh dear. That was a waste of a job. So I guess question now is what's the best way to back fill this spot and perhaps I can unblock the pipe they run to later with some sort of rod as they don't appear to be crushed but do fill up when i run the hose in them