Advice please on possibly cutting a trench through up to a fifth of this tree's roots please...

Is there a way to do this without seriously injuring this wonderful tree?

Where on the internet or in the real world can I get advice?
What questions should I be asking?

We need to remove the rainwater from some roofs which was inserted into the sewer a few decades ago and hence floods the septic tank whenever it rains and leads to a pong in the lane!

video: https://youtu.be/b3GVYTpZEw4

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