Drainage question

My mother’s section is on a slope, and the houses above hers discharge their stormwater down hill.
She has an open drain, at the top of her section
All the land above hers, used to be bush, she’s been there 70 years or so.
Her section is getting swamped
The council are no help, and when I approached one of the owners above her about it, he told me he was selling and basically FS. ( sold 2 weeks later )
I want to run Novacoil from the open drain, down the side of her section, and into a stormwater drain where her section meets the road.
Thinking 100mm
The section is 1/2 acre.
Any thoughts or suggestions welcomed
Cheers

I had nearly the same issue a while back and found it was a mix of old clay pipes and poor gradient causing backups. Ended up getting some help from https://www.pm247.co.uk/services/plumbing/ and they figured out a better way to sort the fall without having to tear everything up. If you’re not keen on digging, sometimes just rerouting can fix things quicker than expected.