What leafy plant is this?

It definitely isn't celery or coriander.

Feverfew sounds like the closest so far.. But i guess i may have to wait until it flowers!

 

If it's a young feverfew it wouldn't flower this year. Sometimes they form one main stalk, depends on how good their growing situation has been.

 

It is only young, both, you are quite right. It's only popped up a couple of months ago or less time than that - so it's a new young 'un!
It is pretty single stemmed atm.. But its bushing and branching out, i feel it will quickly spread as it has come up (the border one) suddenly and very quickly from nowhere!