Exactly the same rather coarse, spiralling foliage as the normal Rosebay Willowherb, but beautiful white flowers.
No interactions present.
The history of the two generic names looks as if it's fairly complex - perhaps it didn't get resolved earlier because the genus was commonly sunk in Epilobium.
Holub plumped for Chamerion in 1972, but there's a 2011 (postdating Stace) paper by Sennikov that goes the other way. Unfortunately both papers are paywalled so I can't easily evaluate the arguments.
Lat/Lng: 51.3009, -0.9282
OS grid ref: SU748562
In the main walled garden border. Beautiful place, well worth a visit.