Willowherb?

That was my first thought Stera. Yes a weed, it'll produce tiny pink flowers on stems then a stinkload of fluffy, airborne seed. My garden is almost completely left unkempt for wildlife but I pull this up regularly before it flowers

don't think it is willowherb going by the leaf. not sure what it is yet. but I will have a think. possibly a campanula
you say you planted it. did you grow it from seed/cuttings or did you buy plug plants. Are the leaves aromatic like bergamot

I planted “something” last year which I picked up at a local plant sale. It bore large yellow flowers, but didn’t look like this before it flowered I’m sure. Will smell the leaves in the morning and let you know if they’re aromatic.

Leaves not aromatic at all. Stood looking at it this morning and suddenly thought “sod it” and dived in and pulled the whole thing up! As usual I apologised to it as I was doing it - always do!!

Solidago?

Sweet Rocket? Curious now! Please do let it flower before you pull it out! If it is sweet rocket it will be a magnet for butterflies.

Studying it again I think Cottagekaren might be right.

Didn't recognise it first time round and then just suddenly thought Golden Rod - just noticed Karen got there first...

I have it at the same stage, so recognised it immediately. :)