Advice Needed: Soil Stack Hole (With Pics)

Hi Gents

I hope someone here can give me some sound advice.

My soil stack have been moved outside, the old one ran internally and left the property via a rest bend.

This rest bend has now been removed and i'm left with a hole that goes from the outside of my house to the inside.

It is below the DPC.

What can I do to fill this hole between the cavities? I plan to then reinstate the ground work with gravel.

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Fill it dear Henry, dear Henry, dear Henry...

a couple of bricks.........sand and cement=mortar.