Any ideas?

This plant had appeared at the edge of our pond, not in the water but quite near. We have no idea what it is. Neither of us remember planting it but we may well have chucked in some seed or have stuck in a stolen bit of plant from a friends garden. Or it could just have arrived on it's on, that happens quite often.

It is lovely, about 60cm high, multi stemmed with a big flower head on each stem. Leaves are kind of heart shaped coming directly out of the stem and are green but with a slightly jagged edge.

The flowers are about 12cm in diameter and a deep dark blueish purple. Made up from individual flowers that remind me of canterbury bells. My camera struggled with the colour it's actually a lot darker and bluer than this. Kind of the colour of a blue iris.

Does anyone have any idea what it may be? And if so should I take cuttings or collect the seed to grow more?

.....Edit.....

Changed my mind, just looked out the window and it is more purple than blue but definitely a lot darker than this.