Mysterious red

This plant is in Benmore Gardens, beside the ornamental lake. It's almost as tall as I am - so it must be nearing 5' - and it looks like sorrel, only huge. But the leaves seem to me to be the wrong shape - there's one in the bottom right of the photo - and I remain puzzled.

A glorious afternoon in the gardens. There isn't much in flower just now - the colour of the late Spring has faded and the flowers fallen, and the autumn azalea colours are yet to develop - but I saw a wonderful white rhododendron against the blue sky, alive with bees; there are some new tiny waterlilies appearing in the pond (cleared out over the winter); there was this.

Good enough ...