Solved Any Idea what this plant is

I probably would have pulled it out by now.

 

Lots of the willowherb tribe are annuals and pull up quite easily, but annoyingly, some of them look exactly like the annuals, but are perennial and spread by thick white roots. Rosebay willowherb is one of the perennials. They all have an amazing ability to masquerade as something you love, like phlox, especially when young.

 

And in clumps of Michaelmas daisies...

 

And penstemons!

 

Thanks all. 100% its Willow Herb. Its coming out at the weekend.