A lovely thing. But as you have discovered, a bit unruly at times.
Cut it back by all means. Though, this year everything is a bit later than usual. But even so, another crop of flowers should result.
Dig up those clumps that are surplus to requirements and pass them on to friends and neighbours.

As above. In autumn, dig it up, chop the plant into quaters, plant one quater and give the rest to friends

Thanks so much for both replies. I didn't know it could be split. Oh! there are knowledgeable members on this site, who needs a gardening book, eh?