Anyone know what this plant is?

I suspect it's a weed because we didn't plant it (and isn't that the definition of a weed anyway?) but it's not an unpleasant flower, so I'd be interested to know whether it's something that will take over the garden or poison the dog if we don't get it out....

 

It could be Honesty, after the purple flowers silver papery discs appear which you can cut and dry out to be displayed in the house, I have some in my garden and like it, it doesn't take over and is easily controlled.

 

http://www.dog-pictures.co.uk/dog-care-articles/common_plants_poisonous_to_dogs.shtml

The above link is very useful in finding out which plants are dangerous for dogs, some plants are dangerous to specific breeds. This link has them all, hope it helps :D

 

Looks a bit like a young honesty plant to me.