Saponaria

Hmm, I did some google images etc - they all appear low, and this is well over a foot and a half. Are there different cultivars?

Silene dioica

That's it! Thanks Jaynine!

Aka Red Campion - and a very lusty specimen too. The petal shape makes me wonder if it is a hybrid between S dioica and S latifolia (White Campion). Such hybrids do occur, even in nature, quite often. Both are native wildflowers in the British Isles.

Same family as Saponaria. The one Thedecoguy was thinking of is Saponaria officinalis which is the same height as Red Campion.