Pink flowered garden plant (a bit like Hesperis)

Pink flowered garden plant (a bit like Hesperis)

Observation - Pink flowered garden plant (a bit like Hesperis). Description: I found this in my local park the other day and thought initially it was a garden variety of Hesperis but it has an extra petal, so it must be something else - presumably non-native. Any thoughts or guesses would be very much appreciated! Thanks

I found this in my local park the other day and thought initially it was a garden variety of Hesperis but it has an extra petal, so it must be something else - presumably non-native.

Any thoughts or guesses would be very much appreciated!

Thanks