not 100% sure but think it may be one of the willow herbs.

It's one of the Toadflax species....Linaria sp. Pretty plant in yellow, pink and purple but spreads everywhere !!!!

Its Linaria purpurea and is a short lived perennial but does seed freely. You may get a pink form called 'Cannon Went'. The yellow form of toadflax is a different species to this one.

The bees adore this linaria, so if you are happy seeing lots of bees, and are fairly happy with random sprouting, let it seed. It's not a bulky plant so doesn't block others out.

Thanks for all the information everyone. We do have lots of bees in the garden, so maybe this is why! ?