Is it a weed

The difference between a plant and a weed is this: If you want it where it is and like how it looks there...then it is a plant... If you don't want it where it is and wish it would go away it is a weed... What you have pictured is a violet...they make a wonderful ground cover...after blooming their leaves become dark green and they are a lovely heartshape. they will remain at maximum six inches tall...and will die down with frost...they will form clumps up to ten inches wide and spread easily I have about 60 sq feet now covered with a combination of pachysandra, violets and ajuga... I dislike lawns and won't feed and coddle grass (the worst weed of all) so this is why I know about ground covers that work... don't poison the violets...in spring they are a lovely purple mat and some voilets have a wonderful smell.
After taking a good look at your photo...this could also be a campanula... give that a google and see what you can find.
good luck.

Mmmm! It hasn't flowered yet and some of the leaves are as big as my hand??? Jackie x

hmmm...maybe the second thought was the right one...perhaps it's a campanula...lol. maybe put a coin beside a sample for a sense of scale...good luck.