Like many moths, this Triple-stripe Piercer has a very descriptive name. having said that, it has four stripes not three (I guess it has three dark stripes on a pale blotch). As far as the 'Piercer' in its name, I believe that refers to the caterpillar's method of feeding. It is very very small, with a wingspan of less than a centimetre, so with wings folded like this - about half a centimetre long - it's very difficult to even see that it is a moth with the naked eye. The larvae feed on Clover. It is quite common in the UK, although less so in the north.