Hypericum. Not sure which one though

Agree, but not sure which one, either. Known otherwise as St John's Wort.

Yep. That is what it is. Seeds easily, can grow big.
One day I will find another site for mine.

Thank you all!

Lol Fractal_cat there are seedlings springing up all over the place, at least I know what's going on now :o)

This plant was invasive in my garden, kept seeding all over the place, and like Fractal said they can get big. A well-behaved Hypericum is 'Hidcote', evergreen with large 'buttercup' type yellow flowers, and it doesn't seed itself around.

If its all over the place, its most likely Hypericum calycinum... very invasive.

Thanks Jaykaty and Bamboo :)