65mm Square Downpipe to 82mm Round Adapter?

[RBZ5416]

Would this be better in the plumbing section?

The bottom of the square downpipe was mortared into place. This has now cracked & split to reveal the bodge underneath. Sunk into the concrete is a black plastic pipe with an approx 82mm internal diameter. The square downpipe just sits on this so I need a proper solution to connect it. So far I've failed miserably to find an adapter, so can anyone please help with identifying one or coming up with a creative solution?

Best I've come up with so far is trying to find a cap that will fit the round pipe, adding a square to round adapter on the bottom of the downpipe & then cutting a 65mm hole in the cap. But to do that I need to know exactly what size the round pipe is, as access is tricky until I strip it all back. And I don't want to do that until I have the parts to finish the task.

Edit
A bit more digging seems to confirm the the pipe in the ground is likely 82mm with a "socket". This is the closest I've found to my needs, other than it being round & the worst possible colour given it will be on show. Ideally want white or black at a push.

 

[noseall]

Would this be better in the plumbing section?

The bottom of the square downpipe was mortared into place. This has now cracked & split to reveal the bodge underneath. Sunk into the concrete is a black plastic pipe with an approx 82mm internal diameter. The square downpipe just sits on this so I need a proper solution to connect it. So far I've failed miserably to find an adapter, so can anyone please help with identifying one or coming up with a creative solution?

Best I've come up with so far is trying to find a cap that will fit the round pipe, adding a square to round adapter on the bottom of the downpipe & then cutting a 65mm hole in the cap. But to do that I need to know exactly what size the round pipe is, as access is tricky until I strip it all back. And I don't want to do that until I have the parts to finish the task.

Edit
A bit more digging seems to confirm the the pipe in the ground is likely 82mm with a "socket". This is the closest I've found to my needs, other than it being round & the worst possible colour given it will be on show. Ideally want white or black at a push.

82mm? WTF.

 

[RBZ5416]

82mm? WTF.

Kind of my thoughts too, but seems to be one of these.

Just episode 5,379 in my life of "Why is nothing ever standard!?"

Given the cement lasted 20 years, I might just re-bodge it. It'll likely see me out...