Does anyone know how to control/eradicate Ground Elder/Gout Weed? (corrected: was initially Cow Parsley)

That looks like giant hogweed not cow parsnip. It is a nasty invasive and you want to take care in removing it. It contains a phyto toxin. Small plants like the one in the photo are easy to dig up. Use gloves, cover your skin and do it before it seeds.