[Jez]

Very similar comparisons to be made here I think Wig. The stems look square but aren't they do have 5 sides, hairs running down the stems, but I wasn't prepared to break the stem for residue colour I'm affraid. Though what you mention I find interesting! you say the residue is purple now could this possibly be because the flowers colour will be purple if what you have is a Campanula!?! Personally I don't think it's Nettle Leaved Bellflower, I'm leaning towards Clustered Bellflower as Tiggrx mention due to the clump of five flower heads starting to show, looking though a couple of wildflower books none of the pictures show this flower with woody looking stems! That's all I can offer for now. Just hope it does come into flower as I'm curious to know what plant you have!

P.s Flowering time for Clustered Bellflower seems to be about right.