Plant or weed?

[theclockticksslowly]

Tackling my parents overgrown garden and have no idea what this is! It’s growing very well and there’s lots of it!

Looks a bit like saponaria to me but probably a bit tall. Wait till it flowers, work out if you like it. A weed is only a plant you don't want.

However, that amount of growth makes me think that regardless of how lovely it might be it will need to be controlled.

I wondered if it was loosestrife? That can spread but you will gets lots of yellow flowers if it is

Thanks for the suggestions! If I remember correctly from previous years it does have yellow flowers!