I know it's a real puzzle. We also sent a picture to John Simmons (ex curator of the living collections at Kew) he suggests
S. sambucifolia. Looking at Google I think he may be correct, he often comes on some of our weekends away and is an absolute goldmine of information.
It's a lovely plant growing well in woodland conditions to about a metre, I'm hoping it will have got through the winter but there is no sign of it yet
