scarlet pimpernel

I've had a real problem with scarlet pimpernel in my strawberry patch this year, keep pulling it up and it comes back - I'm wondering if it could have come in with the straw that I put down around the plants (bought from local pet shop). Anyone else had this problem?
I did dig all the plants up earlier this year and replant them to try and get rid of the couch grass - so maybe the pimpernel seeds were dormant in the ground and sparked into action by the digging?