at this stage it could be one of a number of plants. crush the leaves are they aromatic?

As it happens I came across the exact same plant whilst out walking the dog half an hour ago. I'm pretty certain it's Tansy which will grow to about 3 feet. A pretty plant but it will take over your garden.

Hi Seaburngirl Thank you for that, yes it smells aromatic like a sage.

might be rayless mayweed [ Matricaria discoides] if it is it produces a flower that looks like the centre of a daisy butv more like green velvet and gets to a height of 6-15".
any taller and Tansy [Tanaceteum vulgare] is a good suggestion from Arbuthnot it will have yellow flowers and gets to 2-4ft in height. Also aromatic when you crush the leaves.