It smells like mint because it is mint. What kind of mint depends on the smell. Next time sil wants an id could he show it the right way up in situ? Upside down in the air is tricky. Behind him is a mega Buddleja, blight of the British urban garden.

Actually the butterfly bush is not particularly large. But the weediness aspect will certainly still be present.

OK, it's tiny.

But I was actually looking at the trunk as thick as my leg, not the volume of the top. It's clearly been hacked back many times.

You have skinny legs.

They're not bad 😉

With that dark purple stem, it is highly likely chocolate mint, my favorite.

The scent would confirm that. But the OP said it smelled of mint. And plenty of mints have dark stems.

Chocolate mint has the darkest stems.

That's because they're made out of chocolate - if you break one off it's good for stirring coffee.

"Chocolate mint has the darkest stems."

That would be debatable :-) As mentioned, there are any number of varieties of mint that have dark, reddish or burgundy stems or rhizomes. Black peppermint might be a better contender for darkest......