Unknown white flower in coastal rock crevice

Unknown white flower in coastal rock crevice

Observation - Unknown white flower in coastal rock crevice - UK and Ireland. Description: ?Crusiaceae Low-growing plant tucked into rock crevice Flower about 10mm diameter with 4 white petal

?Crusiaceae
Low-growing plant tucked into rock crevice
Flower about 10mm diameter with 4 white petals in shape of cross with and inner whorl of green ?petals, 6 stamen