This is a tiny flower that grows amongst short turf on limestone soils and as a result it is a nationally scarce plant as the conditions it needs are rare. It is also difficult to find and very difficult to photograph as it is so small (this is very much a 'could do better' photograph I am using!).
It is a semi-parasitic plant getting some of its nutrients by taking advantage of nearby grasses. That may be the origin of the first part of its name but it is not a member of the toadflax family so where that comes from is anyones guess!
For those of a more sensitive nature who might find bastard toadflax a bit offensive a new name has recently been coined for this plant and it is now also known as stars in the grass because of the five pointed petals that are arranged in a star shape.