It looks like a double-flowered form of Bouncingbet (Saponaria officinalis).
It spreads vigorously from rhizomes and also from abundant seeds. The key
to control is to eliminate the top growth over and over again to deplete the
rhizomes and to rigorously remove any new seedlings.

Bouncingbet has been officially designated as an invasive weed in Colorado
(where my mother lives) which requires it be "either eradicated, contained, or
suppressed" and the suggested control method is to "[h]andpull or dig only
single plants/new infestations when the soil is moist" to ensure you remove
as much of the root system as possible. The recommend herbicide is
Chorsulfuron (brand names: Glean, CORSAIR, Telar) applied at "bolting to
bud growth stage."

Link to Colorado website:
TinyURL.com - shorten that long URL into a tiny URL

Other states have not gone as far as Colorado in restricting this plant, but
it is a problem in Long Island, New York and parts of Michigan. (The
Michigan Nature Conservancy has discovered thay glyphosate does not
offer more than a 50% chance of control, at best.)

The best time to attack a plant like this is just as the flowers are forming,
as the root reserves will be lowest at that time. Too bad it is so very
invasive, as the flowers are pretty and fragrant.


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Oh, thankyou so much! It is a shame as they are lovely little flowers but it's taken over the whole garden. Im not sure if I can get those brands of herbacide in the UK but I will have a look, if nothing else it's good to know what it is and persist in pulling it up and I can start trimming off the top growth too.
Very interesting to know that it's designated as an invasive weed in Colorado too.
Thankyou very much for your help.

-ekh